S8 PRELIMINARY ACCOUNT. [CHAP. 



time it becomes split or cleft horizontally over the greater 

 part of its extent into two leaves, an upper leaf and a lower 

 leaf. In the neighbourhood of the axis of the body, beneath 

 the neural tube, this cleavage is absent (Fig. 8, B ; also Figs. 

 13 — 20), in fact, it begins at some little distance on either 

 side of the axis and spreads thence into the periphery in all 

 directions. It is along the thickened mesoblast that the 

 cleavage takes place, the upper part of the mesoblast uniting 

 ■with epiblast to form the upper leaf, and the lower part with 

 the hypoblast to form the lower leaf. 



In the fundamental folds both leaves are involved, both 

 leaves are folded downwards and inwards, both leaves tend 

 to meet in the middle below; but the lower leaf is folded in 

 more rapidly, and thus diverges from the upper leaf, a space 

 being gradually developed between them (Fig. 8). In course 

 of time the several folds of the lower leaf meet and unite to 

 form an inner tube quite independently of the upper leaf, 

 whose own folds in turn meet and unite to form an outer 

 tube separated from the inner one by an intervening space. 

 The inner tube is the alimentary canal which is subsequently 

 perforated at both ends to form the mouth and anus; the 

 walls of the outer tube are the walls of the body, and the 

 .space between the two tubes is the general " serous cavity," 

 which being subsequently divided into pleural and peritoneal 

 portions, may be spoken of as the pleuroperitoneal cavity. 



Hence the upper (or outer) leaf of the blastoderm, from 

 its giving rise to the body- walls, is called the somatopleure^ ; 

 the lower (or inner) leaf, from its forming the alimentary 

 canal and its tributary viscera, the splanchnopleure^ . 



This horizontal splitting of the blastoderm into a somato- 

 pleure and a splanchnopleure, which we shall hereafter speak 

 of as the cleavage of the mesoblast, is not confined to the region 

 of the embryo, but gradually extends over the whole of the 

 yolk-sac. Hence in the later days of incubation the yolk- 

 sac comes to have two distinct coats, an inner splanchno- 

 pleujic and an outer somatopleuric investment, separable 

 from each other all over the sac. We have seen that, owing 

 to the manner of its formation, the ' embryonic sac ' is con- 

 nected with the 'yolk-sac' by a continual narrowing hollow 



^ Soma, body, pleuron, side. * Sjplanchnie, viscus, ple%iron, side. 



