THE GROWTH OF THE EMBRYO. 107 



soon as the stage of the blastodermic vesicle is reached, and while the vesicle is 

 very small. In such cases the ovum makes a cavity for itself by dissolving away 

 the epithelium and connective tissue at a small spot on the uterine surface, mak- 

 ing a cavity in which it lodges itself. In other cases the trophoblast is developed 

 later and does not appear over the whole of the blastodermic vesicle. The area 

 over which it exists in such cases is called the placental area (compare pages 

 121 and 122). The trophoblast in these forms unites very closely indeed with the 

 surface of the uterus, and the uterine tissues undergo degeneration and resorp- 

 tion. We may regard as the first step toward the production of the placenta 

 proper the disappearance of the trophoblast. Our knowledge of its disap- 

 pearance is incomplete, but it is probable that it is due to a transformation of the 

 cells of the trophoblast, associated with contemporaneous modifications of the 

 chorionic membrane, of which the general result may be said to be formation of 

 the chorionic villi which constitute the foetal portion of the placenta. The 

 modified trophoblastic cells are supposed to enter into the formation of the 

 ectodermal covering of these villi. 



The Growth of the Embryo. 



In all vertebrates there is provision made for the nutrition of the embryo, 

 the development being strictly of the embryonic type. In most cases this pro- 

 vision consists in a sort of yolk material, but in the placental mammals the pro- 

 vision is made by means of the placenta from the uterus of the mother. In 

 either case the embryo has only to assimilate the food which is already more or 

 less prepared for it, and we find that in all vertebrates there is an extremely rapid 

 growth of the embryo. In amniota we have a marked distinction between the 

 embryo proper and its so-called appendages, the yolk-sac, chorion, amnion, and 

 allantois. These appendages are all ultimately sacrificed for the benefit of the 

 embryo, and in mammals, except for a portion of the allantois which is retained 

 within the body of the embryo as the anlage of the bladder, these appendages are 

 ultimately cast off altogether, and take no part in the construction of the child 

 after birth. We note, in fact, as we ascend the vertebrate series, an increasing 

 tendency to give the embryo prominence and differentiate it more decisively 

 from the embryonic appendages. This becomes so marked in the higher verte- 

 brates that we speak of the growth of the embryo almost as a separate thing from 

 the growth of the appendages. 



The embryo, when its differentiation commences, lies as a small area upon 

 the surface of the ovum. By the growth of the tissues of this embryonic region, 

 the embryo at once begins to enlarge, and as it enlarges we see that it outstrips 

 the extra-embryonic structures with which it is associated, and first the head 



