INTRODUCTION 9 



The dorsal part of the mesoderm which lies on either side of 

 the middle line in all cases early becomes metamerically 

 segmented to form a longitudinal series of mesodermic somites or 

 protovertebrce, which lose their cavities (cf. Fig. 6, A and B), and are 

 concerned in the formation of the vertebral column, body-muscles, 

 and part of the urinogeni tal apparatus. This primary segmenta- 

 tion must be distinguished from the segmentation which appears 

 later, on the formation of the vertebral column, ribs, spinal nerves, 

 &c. The ventral parts of the mesoderm are known as the lateral 

 plates ; in Ainphioxus alone do they exhibit segmentation. 



As a general rule, a thickened disc-shaped region can be 

 recognised at a certain stage of development on the dorsal pole of 

 the oosperm : this is the so-called embryonic area, on which the 

 first indications of the body are seen (Fig. 4). This region 

 gradually sinks into the underlying yolk, from which it becomes 

 constricted off all round by the formation of furrows, and conse- 

 quently the connection of the body- rudiment with the ventral 

 yolk-sac (by the vitcllo -intestinal duct] is gradually reduced in size, 

 and when the yolk is eventually entirely absorbed disappears 

 altogether (Fig. 8, f). 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. 14). 



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 (Eutlicria^) 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 Reptiles, 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, an important vascular connection 

 takes place between the mother and foetus by means of the 



