IV.] SUMMARY. 95 



from which it springs is known as the intermediate 

 cell mass. We shall return to them immediately. 



SuiUUiary. The most important changes then which 

 take place during the iirst half of the second day are, 

 the closure of the medullary folds, especially in the 

 anterior part, and the dilatation of the canal so formed 

 into the first cerebral vesicle ; the establishment of a 

 certain number of mesoblastic somites ; the elevation of 

 the head from the plane of the blastoderm ; the forma- 

 tion of the tubular heart and of the great blood-vessels ; 

 and the appearance of the rudiment of the Wolffian 

 duct. 



It is important to remember that the embryo of which 

 we are now speaking is simply a part of the whole 

 germinal membrane, which is gradually spreading over 

 the surface of the yolk. It is important also to bear in 

 mind that all that part of the embryo which is in front 

 of the foremost somite corresponds to the future head, 

 and the rest to the neck, body and tail. During this 

 period the head occupies about a third of the whole 

 length of the embryo. 



