II.] THE BODY CAVITY. 41 



to an increase in the middle layer or mesoblast, while 

 at the same 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. 9, B ; also Figs. 24, 34), in fact, 

 it begins at some little distance on either side of the 

 axis and spreads thence into the periphery in all direc- 

 tions. It is along the 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. 9). 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 which from its mode of formation is 

 clearly lined by hypoblast 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 hody or pleuivperitoneal cavity. 



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

 from its giving rise to the body-walls, is called the 

 somatoplewre ' ; the lower (or inner) leaf, from its form- 



1 Soma, body, pleuron, side. 



