THE DEVELOPMENT OP THE MOLE. 127 



Posteriorly the pleuro-peritoneal cavities are each continuous 

 with the body cavity contained between the diverging folds of 

 the somatopleure and splanchnopleure. 



The Primitive Streak. — During Stages e and f the relations 

 of the primitive streak are almost exactly similar to those 

 described for Stage d (No. 8)^ the only diflference being the 

 extension of the medullary folds backwards round the front 

 end of the primitive streak (fig. 18). The lumen of the 

 neurenteric canal disappears, but the point where it originally 

 existed is shown by the fusion of the epiblast, hypoblast, noto- 

 chordj and primitive streak mesoblast at the front end of the 

 latter (Stage j, fig. 50). 



In my paper. No. 8, I endeavoured to prove the mesoblast 

 of the primitive streak did not extend beyond the point where 

 the neurenteric canal was situated, and I showed that over the 

 whole of that part of the embryo situated anterior to the 

 primitive streak, mesoblast was formed from the hypoblast 

 ("hypoblastic mesoblast"). 



Now if this be true, it follows that the mesoblast of the primi- 

 tive streak takes no part in the formation of the body of the 

 embryo anterior to the neurenteric canal, and that the growth 

 of the embryo is caused by a multiplication of cells anterior 

 to the primitive streak. 



The mesoblast of the primitive streak is, however, a con- 

 siderable and hitherto a constantly increasing mass, and it 

 extends backwards and outwards beyond the embryonic area. 

 It thus occupies the position where eventually the allantois is 

 formed, and it is, in fact, the primitive streak mesoblast which 

 forms the walls of that organ. 



During the stages now under discussion (b — j) the primitive 

 streak becomes partially — almost entirely — divided into two 

 portions, an anterior and a posterior portion. The division is 

 caused by the formation of two pits — (1) a dorsal pit which 

 eventually gives rise to the anus, and (2) a ventral pit which 

 projects upwards and backwards into the primitive streak, and 

 forms the cavity of the allantois (figs. 35 and 50). 



These two pits constrict the blastoderm and partially divide 



