INTRODUCTION 9 



vitello-intestinal duct) is reduced in size, and when the .yolk is 

 eventually entirely absorhed, disappears altogether (Fig. 8, j-). In 

 the higher Vertebrates (Reptiles, Birds, and Mammals) folds of the 

 somatopleure arise externally to these furrows, and are known 

 respectively as the head, tail, and lateral folds ; these gradually 

 grow upwards and eventually unite with one another dorsally so 

 as to form a membranous, dome-like sac, the amnion (Fig. 8) 

 which encloses the embryo and contains a fluid (liquor amnii). 



Owing to the presence of this structure the above-named 

 Vertebrates are usually distinguished as Amniota from the 

 Anamnia (Fishes and Amphibians), in which no amnion is 

 developed (p. 13). 



A network of blood-vessels becomes developed over the yolk- 

 sac, which may therefore serve as an organ of respiration as 

 well as of nutrition. But in the higher Mammals this func- 

 tion is only a very subsidiary one, as at a very early stage a 

 vascular sac-like outgrowth, the allantois (Fig. 8), arises from 

 the hinder part of the intestine (i.e., from the splanchnopleure). 

 This serves not only for respiration, but also for the reception of 

 excretory matters derived from the primitive kidney. It is also 

 present in Amphibians, but in them remains small, and does not 

 extend beyond the body cavity of the embryo; while in the 

 Amniota it gradually increases in size and grows round the embryo 

 as a stalked vesicle, which in Eeptiles, Birds, and Monotremes 

 comes to lie close beneath the egg-shell and acts as an efficient 

 respiratory organ during the rest of the embryonic period. 

 Towards the close of this period the allantois gradually undergoes 

 more or less complete reduction. 



In the higher Mammalia, however, an important vascular con- 

 nection takes place between the mother and foetus by means of the 

 allantois. The latter becomes attached to a definite region of 

 the uterine wall, and from it vascular processes or villi arise, so 

 that the foetal and maternal blood-vessels come into very close 

 relations with one another. Thus an allantoic placenta is 

 formed, which serves both for the respiration and nutrition of the 

 foetus (Fig. 9). As an allantoic placenta is not developed in 

 Monotremes and is only slightly indicated amongst Marsupials, 

 these forms are distinguished as Aplacentalia from the higher 

 Mammals, or Placentalia (p. 14). 



The following important points must be noted as regards the 

 structure of the Vertebrate body. After the main organs have ap- 

 peared, a smaller dorsal neural tube and a larger ventral visceral 

 tube extend longitudinally through the body, and between the two 

 is a rod-like supporting structure, the notochord (p. 5), which 

 arises as an axial thickening of the primary hypoblast and forms the 

 primitive skeletal axis : it is usually replaced by a vertebral column- 

 consisting of centra and arches, at a later stage of development 

 (Fig. 7). All these are median in position, and the body is thus 



