68 THE FIRST DAT. [CHAP. 



owing to the thickness of the medullary folds ; as 

 these folds slope away outwards on either side, so 

 the opacity gradually fades away in the pellucid area. 

 There is present at the sides no sharp line of demarca- 

 tion between the body of the embryo and the rest of 

 the area; nor will there be any till the lateral folds 

 make their appearance ; and transverse vertical sections 

 shew (Fig. 21) that there is no break in the mesoblast, 

 from the notochord to the margin of the pellucid area, 

 but only a gradual thinning. 



During the latter period of the day, however, the 

 plates of mesoblast on either side of the notochord 

 begin to be split horizontally into two layers, the one 

 of which attaching itself to the epiblast, forms with 

 it the somatopleure (shewn for a somewhat later stage 

 in Fig. 24), while the other, attaching itself to the 

 hypoblast, forms with it the splanchnopleure. By 

 the separation of these two layers from each other, 

 a cavity (Pp), containing fluid only, and more con- 

 spicuous in certain parts of the embryo than in others, 

 is developed. This cavity is the beginning of that 

 great serous cavity of the body which afterwards becomes 

 divided into separate cavities. We shall speak of it as 

 the pleuro-peritoneal cavity. 



This cleavage into somatopleure and splanchno- 

 pleure extends close up to the walls of the medullary 

 canal, but close to the medullary canal a central or 

 axial portion of each plate becomes marked off by 

 a slight constriction from the peripheral (Fig. 24), and 

 receives the name of vertebral plate, the more external 

 mesoblast being called the lateral plate. The cavity 

 between the two layers of the lateral plate rapidly 



