COMPAKATIVE EMBRYOLOGY 801 



organs. The period of organ formation is succeeded by a period of 

 histological differentiation, the result of which is a fully formed but 

 small organism. Its subsequent groAvth is accompanied by changes 

 which are spoken of as metamorphosis, leading to the adult animal. 

 Summarizing the stages in the embryological history of an organism, 

 we find : gametogenesis, fertilization, cleavage, germ layer formation, 

 organ formation or organogenesis (Fig. 110), histological differentia- 

 tion or histogenesis. The ontogenetic cycle may be completed if we 

 carry the history of development on through adolescence, adulthood, 

 and old age. 



The first problem to which the developing embryo must be adapted 

 is that of a store of food to enable it to grow until it can obtain food 

 for itself. At the same time it must be protected from the adversities 

 of the environment; these are of chemical and physical nature and 

 also include other organisms which would prey upon it for food. The 

 first of these problems is solved by the storage of a sufficient amount 

 of yolk (Figs. 287 and 288) in the maturing egg to carry the embryo 

 through its period of helplessness. The duration of this period de- 

 pends upon the habitats in which the embrj^os develop, and hence there 

 are various kinds of yolk with respect to form and deposition in the 

 egg. 



To solve the second problem, that of protection, many devices are 

 to be seen throughout the animal kingdom. Conspicuous among them 

 are the types of membrane, including shells and horny structures 

 which protect embryos from physical harm and regulate the chem- 

 ical exchanges with the surrounding medium. There are also many 

 delicate adjustments, as for example, the oil droplets which cause some 

 eggs to have a lighter specific gravity and thus to float at the surface 

 of the water w^here certain forms of animals cannot so readily destroy 

 them. Egg membranes are usually classified into three groups ac- 

 cording to their origin. Those formed from the egg itself are spoken 

 of as "primary membranes," and of them the vitelline is an exam- 

 ple. Those formed from the follicular cells are "secondary mem- 

 branes," and of these the true chorion is an example, and finally 

 those formed from the wall of the oviduct or uterus are "tertiary 

 membranes," of Avhich the albumen and shell of the bird egg are 

 examples. 



The amount and kind of distribution of yolk are to no small extent 

 responsible for the types of cleavage found in the animal kingdom. 

 Without explaining this relation in detail, we may say that there are 







