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round and meet in the middle line. Anterior to the headfold is a second fold parallel to it, which is the commencing amnion. The medullary canal is bounded by its two folds or longitudinal elevations, lamine dorsales, which are folds consisting entirely of cells of the epiblast: these grow up and arch over the medullary groove (fig. 446) till after some time they coalesce in the middle line, converting it from an open furrow into a closed tube-the neural canal or the primitive cerebro-spinal axis. Over this closed tube, the walls of which consist of more or less cylindrical cells, the superficial layer of the epiblast is now continued as a distinct membrane.

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Fig. 449.-Embryo chick (30 hours), viewed from beneath as a transparent object (magnified). pl, outline of pellucid area; FB, fore-brain, or first cerebral vesicle: from its sides project on, the optic vesicles; 80, backward limit of somatopleure fold, "tucked in" under head; ", head-fold of true amnion; a', reflected layer of amnion, sometimes termed "false amnion;" sp, backward limit of splanchnopleure folds, along which run the omphalomesaraic veins uniting to form h, the heart, which is continued forwards into ba, the bulbus arteriosus; d, the fore-gut, lying behind the heart, and having a wide crescentic opening between the splanchnopleure folds; HB, hind-brain; MB, midbrain; pv, protovertebræ lying behind the fore-gut; me, line of junetion of medullary folds and of notochord; ch, front end of notochord; vpl, vertebral plates; pr. the primitive groove at its caudal end (Foster and Balfour).

The union of the medullary folds or lamina dorsales takes place first about the neck of the future embryo; they soon after unite over the region of the head, while the closing in of the groove progresses much more slowly towards the hinder extremity of the embryo. The medullary groove is by no means of uniform diameter throughout, but even before the dorsal laminæ have united over it, is seen to be dilated at the anterior extremity and obscurely divided by constrictions into the three primary vesicles of the brain.

The part from which the spinal cord is formed is of nearly uniform calibre, while towards the posterior extremity is a lozenge-shaped dilatation, sinus rhomboidalis, which is the last part to close in (fig. 447).

Whilst the changes which have been described are taking place in the area pellucida, which has en

larged to a certain extent, the area opaca has considerably extended. The hypoblast and mesoblast have also been prolonged laterally, not by mere extension, but also from the germinal wall, which is the thickened edge of the blastoderm, together with formative cells of the yelk; on each side of the notochord and medullary canal, the mesoblast remains as a longitudinal thickening.

It now however splits horizontally into two layers or lamina (parietal and visceral): of these the former, when traced out from

Fig. 450.-Transverse section through dorsal region of embryo chick (45 hrs.). One half of the section is represented: if completed it would extend as far to the left as to the right of the line of the medullary canal (Me). A, epiblast; C, hypoblast, consisting of a single layer of flattened cells; Mc, medullary canal; Pe, protovertebra; Wd, Wolffian duct; So, somatopleure; Sp, splanchnopleure; pp, pleuro-peritoneal cavity; ch, notochord; ao, dorsal aorta, containing blood cells; v, blood-vessels of the yolk-sac (Foster and Balfour).

the central axis, is seen to be in close apposition with the epiblast and gives origin to the parietes of the trunk, while the latter adheres more or less closely to the hypoblast, and gives rise to the serous and muscular walls of the alimentary canal and several other parts (fig. 450).

The united parietal layer of the mesoblast with the epiblast is termed Somatopleure, the united visceral layer and hypoblast, Splanchnopleure. The space between them is the pleuroperitoneal cavity, which becomes subdivided by subsequent partitions into pericardium, pleura, and peritoneum.

The splitting of the mesoblast extends almost to the medullary canal, but a portion on either side (p. v. fig. 450) remains undivided, the vertebral plate. The divided portion is known as the lateral plate. The longitudinal thickening of the vertebral plate is seen after awhile to be divided, at right angles to the

medullary canal by bright transverse lines into a number of square segments. These segments, which are the surface appearance of cubes of mesoblast, are the mesoblastic somites or protovertebræ. The first three or four of these protovertebræ make their appearance in the cervical region, while one or two more are formed in front of this point: and the series is continued backward till the whole medullary canal is flanked by them (fig. 449). That which is first formed corresponds to the second cervical vertebræ. From these somites the vertebræ and the trunk muscles are derived.

Head and Tail Folds. Body Cavity.-Every vertebrate

Fig. 451.-Diagrammatic longitudinal section through the axis of an embryo. The head-fold has commenced, but the tail-fold has not yet appeared. F'So, fold of the somatopleure; FSP, fold of the splanchnopleure; the line of reference, FSo, lies outside the embryo in the "moat," which marks off the overhanging head from the amnion; D. inside the embryo, is that part which is to become the fore-gut; FSo and Fsp, are both parts of the head-fold, and travel to the left of the figure as development proceeds; PP, space between somatopleure and splanchnopleure, pleuro-peritoneal cavity; Am, commencing head-fold of amnion; V, neural canal; Ch, notochord; Ht, heart; 4, B, C, epiblast, mesoblast, hypoblast (Foster and Balfour.)

animal consists essentially of a longitudinal axis (vertebral column) with a neural canal above it, and a body-cavity (containing the alimentary canal) beneath.

We have seen how the earliest rudiments of the central axis and the neural canal are formed; we must now consider how the general body-cavity is developed. In the earliest stages the embryo lies flat on the surface of the yelk, and is not clearly marked off from the rest of the blastoderm: but gradually the head fold or crescentic depression (with its concavity backwards) is formed in the blastoderm, limiting the head of the embryo; the blastoderm is, as it were, tucked in under the head, which thus comes to project above the general surface of the membrane: a similar tucking in of blastoderm takes place at the caudal extremity, and thus the head and tail folds are formed (fig. 452).

Similar depressions mark off the embryo laterally, until it is

completely surrounded by a sort of moat which it overhangs on all sides, and which clearly defines it from the yelk.

This moat runs in further and further all round beneath the overhanging embryo, till the latter comes to resemble a canoe turned upside-down, the ends and middle being, as it were,

Fig. 452.-Diagrammatic section showing the relation in a mammal between the primitive alimentary canal and the membranes of the ovum. The stage represented in this diagram corresponds to that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the allantois; c, the villous chorion; a, the amnion; a', the place of convergence of the amnion and reflexion of the false amnion a" a", or outer or corneous layer; e, the head and trunk of the embryo, comprising the primitive vertebræ and cerebro-spinal axis; i, i, the simple alimentary canal in its upper and lower portions. Immediately beneath the right hand i is seen the futal heart, lying in the anterior part of the pleuro-peritoneal cavity; v, the yolk-sac or umbilical vesicie; vi, the vitello-intestinal opening; u, the allantois connected by a pedicle with the anal portion of the alimentary canal. (Quain.)

decked in by the folding or tucking in of the blastoderm, while on the ventral surface there is still a large communication with the yelk, corresponding to the well or undecked portion of the

canoe.

This communication between the embryo and the yelk is gradually contracted by the further tucking in of the blastoderm from all sides, till it becomes narrowed down, as by an invisible

constricting band, to a mere pedicle which passes out of the body of the embryo at the point of the future umbilicus.

The downwardly folded portions of blastoderm are termed the visceral plates.

Thus we see that the body-cavity is formed by the downward folding of the visceral plates, just as the neural cavity is produced by the upward growth of the dorsal laminæ, the difference being that, in the visceral or ventral laminæ, all three layers of the blastoderm are concerned.

The folding in of the splanchnopleure, lined by hypoblast, pinches off, as it were, a portion of the yelk-sac, enclosing it in the body-cavity. This forms the rudiment of the alimentary canal, which at this period ends blindly towards the head and tail, while in the centre it communicates freely with the cavity of the yelk-sac through the canal termed vitelline or omphalo-mesenteric duct.

The yelk-sac thus becomes divided into two portions which communicate through the vitelline duct, that portion within the body giving rise, as above stated, to the digestive canal, and that outside the body remaining for some time as the umbilical vesicle (fig. 453, ys). The hypoblast forming the epithelium of the intestine is of course continuous with the lining membrane of the umbilical vesicle, while the visceral plate of the mesoblast is continuous with the outer layer of the umbilical vesicle.

All the above details will be clear on reference to the accompanying diagrams.

At the posterior end of the embryo chick, when the amniotic fold is commencing to be formed, and the hind fold of the splanchnopleure has commenced, there remains for a time a communication between the neural canal and the hind gut, which is called the neurenteric canal. It passes in at the point where the notochord falls into the primitive streak. The anterior part of the primitive streak becomes the tail swelling, the posterior part atrophies, and the corresponding lateral part of the blastoderm forms part of the body-wall of the embryo. The anterior part of the medullary canal having been completely roofed in; the foremost portion undergoes dilatation, and a bulb, or first cerebral vesicle results. From either side of this dilatation a process, the cavity of which is in communication with it, is separated off; these processes are the optic vesicles. Behind the first cerebral vesicle two other vesicles now arise, and at the posterior part of the head two small pits, the