GROWTH OF THE EMBRYO AND SEPARATION OF THE YOLK. 



49 



right angles to the long axis of the vesicle. The size of the shield is about the 

 same in all mammals which have been heretofore studied. 



Growth of the Embryo and Separation of the Yolk. 



In all vertebrates the development is strictly of the embryonic type, and 

 accordingly there is made for the nutrition of the embryo some special provision, 



?r 



FIG. 15. TRANSVERSE SECTION THROUGH THE PRIMITIVE STREAK OF AN EMBRYO MOLE. 



EC, Ectoderm. En, Entoderm. mes, Mesoderm. p.gr, Primitive groove. Pr, Primflfve streak. (After W. 



Heape.) 



which in most cases consists of a stock of yolk material; but in the placental 

 mammals the provision is made by means of the .placenta for the transfer of 

 nutriment directly from the mother. In either case the embryo has merely to 

 assimilate the food already more or less prepared for it. It is perhaps owing to 

 these provisions that the growth of the vertebrate embryo is extremely rapid. In 

 the amniota there is a fundamental distinction between the embryo proper and its 



bi 



FIG. 16. DIAGRAMS TO ILLUSTRATE THE SEPARATION OF THE EMBRYO FROM THE YOLK. 

 bl, Blastopore. h, Head of embryo. Ach, Archenteron or entodermal cavity, ec, Ectoderm. 



so-called appendages the yolk-sac, chorion, amnion, and allantois. The append- 

 ages are all finally sacrificed for the benefit of the embryo, and in mammals, except 

 for a portion of the allantois retained in the body as the anlage of the bladder, 

 the four 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 verte- 



4 



