268 



Mr. G. A. Drew on the 



embryo, but never forms a distinct invagination. At most 

 it is only slightly concave when viewed from above, and soon 

 becomes arched outward. These changes are accompanied 

 by the formation of the mid-gut, a term that is not intended 



Fiff. 1. 



. k w^'SwbilifcwSl-, 



External appearance of an embryo of tortj-four hours. 

 ac, apical cilia ; bl, blastopore ; x, ventral depression. 



to include the ventral tube, which is of unknown origin. 

 From the beginning it is connected with the ventral tube, 

 formed, as it were, as a continuation of the anterior extremity 

 of the ventral tube. 



