VERTEBRATE EMBRYOLOGY. The science of embry ology (q.v.) had its first beginnings in the study of the Verte brata (q.v.), the group that includes those forms of life whose ' eggs and breeding habits naturally first attracted attention, and even to-day the mass of known embryological detail relating to vertebrates far exceeds that relating to any other phylum. Fur ther there is no phylum of the animal kingdom which shows in so varying degrees the modifying influence of such factors as amount of yolk in the egg, external environmental conditions, etc.
The Zygote.—The vertebrate, like most animals, begins its existence as a single cell, the zygote or fertilized egg, formed by the fusion of two gametes, derived one from each parent. The zygote possesses in itself all the specific peculiarities of the com plete individual of its species. To human observation, however, the zygotes of different animals do not exhibit any of the peculiari ties differentiating the adults. Such peculiarities as they do pre sent are in such comparatively trivial characters as size, shape, colour. Otherwise each zygote is to all appearance simply a typical cell with cytoplasm and nucleus. The superficial differ ences have to do mainly with adaptive features enabling the young individual to remain for a more or less prolonged period within the shelter of an egg-shell. This is rendered possible in the first instance by the zygote possessing in its cytoplasm a store of yolk—highly concentrated food-material—which provides it with subsistence. The greater the amount of this yolk-capital stored away in the zygote, the greater its size : there is a rough proportion between size of egg and quantity of yolk. Thus in Amphioxus the zygote has a very minute trace of yolk in its cytoplasm and its diameter is about o.i mm.: in the extinct bird Aepyornis of Madagascar, judging from the size of the shell, the zygote may have been as much as 16o mm. in diameter.
In the Mammalia of the most ancient type (Monotremata, q.v.), which still lay their eggs, these are large and richly yolked (Echidna 3.5 mm., Ornithorhynchus 2.5 mm.), and the young
pass through the early development within the egg-shell.
In the ordinary modern mammal, on the other hand, the egg is not laid in the ordinary sense. The zygote is retained within the uterus and there proceeds with its development, absorbing such nourishment as it requires from the mother. The store of yolk, no longer necessary, has disappeared and the zygote has reverted to the small size of from o•i mm. to 0.3 mm. in diameter.
Peculiarities of colour are often due to the yolk, e.g., orange yellow in the case of birds, salmon-pink in Lepidosiren, green in Amia. Yolk is not however the only cause of coloration of the vertebrate zygote. Particularly among the Amphibia, where the egg develops under conditions of exposure to the harmful influ ence of daylight, the superficial layer of protoplasm shows the peculiar "upset" of its metabolism which results in dark brown or black melanin pigment, thus producing a protective, light proof shelter over the deeper protoplasm. This is well seen in the black eggs of the ordinary frogs and toads.
It will be borne in mind that the technical term zygote ex presses the unicellular stage arising from the fusion of the two gametes. As the male gamete or spermatozoon is of quite insig nificant bulk as compared with the macrogamete (unfertilized egg), the obvious features described for the zygote—such as size and colour—have been taken over by it from the macrogamete. The provision of a supply of capital in the form of yolk upon which the individual' can subsist during its early stages is correlated with the fact that during these early stages it lives within the shelter of more or less elaborate protective envelopes. Such are seen in simple form in an ordinary frog, where the egg during its passage down the oviduct is coated with a thin layer of secretion possessing the quality of swelling enormously in bulk when placed in contact with water, the result being the familiar frog-spawn, where each egg lies in the centre of a sphere of clear jelly com posed of the greatly swollen layer of oviducal secretion.