vin DEVELOPMENTAL ADAPTATIONS 465 



majority of Vertebrates during early stages of development. In cases 

 where the embryo lies in contact with maternal tissues the respiratory 

 exchange takes place ultimately, through the thin intervening layers 

 of fluid or envelope, between the blood circulating in the vitelline 

 network and that circulating in the oviducal lining of the mother. 

 In this way all the necessary preliminary conditions are provided 

 for the evolution of a placenta, and as will be shown later these 

 conditions are actually taken advantage of in some cases and a 

 simple yolk-sac placenta is formed. 



In the more highly developed types of yolk-sac the splanchnic 

 mesoderm which surrounds the vitelline vessels sprouts inwards, 

 forming irregular vascular septa which project into the yolk-sac. 

 This modification, which brings about a great increase in the 

 assimilatory surface, reaches such a development in Birds that 

 towards the end of incubation these ingrowths form an irregular 

 meshwork of vascular trabeculae traversing the whole of the yolk 

 right to its centre. 



Eventually the yolk, whether in the form of a yolk-sac or a mass 

 of heavily yolked cells, is enclosed within the ventral wall of the 

 body. In the holoblastic Vertebrates this comes about as already 

 indicated by the simple spreading of the blastoderm over the surface 

 of the yolk so as completely to enclose it. In the Fowl the spreading 

 of the blastoderm, and its derivatives the endoderm and mesoderm, 

 round the yolk is never quite completed, there remaining a small 

 circular patch at which the yolk is separated from the albumen 

 only by the remains of the vitelline membrane (cf. Fig. 215, v.m). 



Further in the Amniota the region of somatopleure bounding 

 the coelomic space in which the yolk-sac lies becomes converted 

 into amnion and serous membrane (cf. Fig. 215, A), and is eventually 

 cast off, playing no part in the formation of the definitive body-wall. 

 The yolk thus lies outside the limits of the definitive body -wall, 

 projecting through the umbilical funnel which is bounded all round 

 by the stalk of the amnion. Eventually, shortly before hatching, 

 the edges of the umbilical opening are drawn over the yolk-sac in 

 a manner which will be described later (see p. 475). In Lacerta 

 vivipara in which the yolk-sac is reduced the remains of it are 

 simply cast off according to Strahl. 



The most remarkable of the excrescences adaptive to an 

 embryonic existence are the organs known as Amnion and Allantois 

 — portions of the embryonic body which become greatly hyper- 

 trophied and perform important functions during embryonic life 

 but which are eventually, for the most part, shed about the time 

 of birth or hatching and play no part in the formation of the body 

 of the adult. 



Amnion. — The most nearly primitive subdivision of the Amniota 

 is the group Eeptilia and we accordingly turn to it and more 

 especially to the Ghelonia, which have been worked out by Mitsukuri 

 (1891), to provide a foundation for our description. 



VOL. II 2 h 



