CHAPTER XVI.

EMBRYOLOGY AND CLASSIFICATION.

THE investigation of the structure and gradual growth of the ovarian egg is so laborious that it will be many years before we can hope to have a complete picture of all its phases. The apparatus required for the task is very complicated, and a long training is necessary merely to prepare the student for the use of his instruments. A superficial familiarity with the microscope gives no idea of the exhausting kind of labor which the naturalist must undergo who would make an intimate microscopic study of these minute living spheres. The glance at the moon, or at Jupiter's satellites, which the chance visitor at an observatory is allowed to take through the gigantic telescope, reveals to him nothing of the intense concentrated watching by which the observer wins his higher reward. The nightly vision of the astronomer, revealing myriad worlds in the vague nebulous spaces of heaven, is not for him; he must take the great results of astronomy for granted, since

no man capable of original research has the time to prepare for the uninitiated the attendant circumstances essential to his more difficult investigations, or to train their eyes to see what he sees. So is it also with the microscopic observer; the deeper insight he has gained by long training in steadiness of hand and eye, as well as in the concentration of intellect that makes the brain work harmoniously with them, he cannot communicate. He may interest and amuse his friends and visitors with some easy exhibition of specimens under the microscope; he may open the door into the laboratory of Nature, but he cannot invite them to cross the threshold or to enter in with him. I think people are not generally aware of the difficulty of microscopic observation, or the amount of painful preparation required merely to fit the organs of sight and touch for the work. In old times men prepared themselves with fast and vigil for entrance into the temple; and Nature does not open her sanctuary without exacting due penance from her votaries. It seems an easy matter for a man to sit down and look at objects through a glass which enlarges everything to his vision; but there are subjects of microscopic research so obscure that the student must observe a special diet before undertaking his investigation, in order that even the beating

of his arteries may not disturb the steadiness of his gaze, and the condition of his nervous system be so calm that his whole figure will remain for hours in rigid obedience to his fixed and concentrated gaze.

After these remarks I trust I shall not be misunderstood by those who have been working in the field of microscopic investigation, and for whose persevering devotion no one can feel a deeper reverence than I do, if I add that there is as yet hardly a beginning in the study of the egg during its growth, and anterior to the formation of the germ. Since Embryology became a science, the great aim of students in that department has been to demonstrate the uniform structure of the egg in all animals, and investigators have limited their observations to that stage of the ovarian egg during which it appears in all animals as a perfect cell. But a new field now opens before us, requiring a careful survey of every stage of growth of the egg, from its first formation to the period when a well-defined germ is developed. The growth of the egg during this period requires to be studied as minutely through all its changes, and in the various combinations of its constitutive elements, as the germ itself has been in its later transformations. Here again, in this later phase, another field presents itself equally new and

full of promise. Embryologists have generally considered their work as complete when they have traced the new being to a point at which it resembles somewhat any of the members of the natural group to which it belongs. The process by which the gradual completion of the whole frame is attained has been assumed to be one of little interest, hardly deserving the careful scrutiny of the embryologist; while the zoologist has also overlooked, or regarded as of little importance, the differences which still distinguish the young from the adult, even after its typical characters are perfectly distinct. Yet naturalists might have taken a hint from one class of Vertebrates long known for their peculiar metamorphoses, and which show how important are the facts to be learned from these early stages in the life of any animal.

More than a century ago Roesel, in his masterly work on the Frogs and Toads of Germany, represented the mode of reproduction and growth of these animals with a remarkable degree of accuracy, and this subject has since been traced with additional precision and minuteness by Rusconi, Von Siebold, and Funke. Notwithstanding this, no special application has yet been made of the results of these investigations to the classification of these animals, beyond the general recognition that the caudate Batrachians, with

permanent external gills, rank lower than the Salamanders, which lose their gills in the adult condition, while these again are inferior to the Frogs and Toads, in which the tail also is resorbed before the animal completes its growth. But the comparison of the higher and lower Batrachians should not stop here. A more extensive examination shows that the Tadpole begins. as an elongated body, not only without legs, but also without external gills, and that it passes to a branchiate condition, with more or less developed legs, before it loses the gills, while there are various modes of development of the limbs themselves, various phases in the formation of the tail, in its growth and resorption; various phases also in the formation of the fingers, up to their final separation, in those which are destitute, in their adult condition, of any web between them. This gradation is so complete, that if we follow all the phases of development of the several representatives of this class, so common everywhere in our temperate zone, we cannot fail to perceive that the changes these animals undergo during their growth furnish a complete scale; and if we now compare this scale with one founded upon the various degrees of structural complication in the adult representatives of the class, we find that these two series agree perfectly; so that Nature herself