30 



PRELIMINARY ACCOUNT. 



[CHAP. 



fore no lateral limits to the body of the embryo as distinguished from the 

 blastoderm. 



Incidentally it shews the formation of the medullary groove by the rising 

 up of the laminae dorsales. Beneath the section of the groove is seen the 

 rudiment of the notochord. On either side a line indicates the cleavage of 

 the mesoblast just commencing. This cleavage, it will be seen, does not exist 

 in the more central parts of the embryo. 



In C, which represents a vertical longitudinal section of later date, both 

 head-fold (on the right) and tail-fold (on the left) have advanced considerably. 

 The alimentary canal is therefore closed in both, in front and behind, but is in 

 middle still widely open to the yolk y below. Though the axial parts of the 

 embryo have become thickened by growth, the body-walls are still thin; in 

 them however is seen the cleavage of the mesoblast, and the divergence of 

 the somatopleure and splanchnopleure. The splanchnopleure both at the head 

 and at the tail is folded in to a greater extent than the somatopleure, and 



PP-1-,::. 



E 



It 



^ 



