Plate IV. 

 Figs. 17 and 18. Loiigitiuliiial sections throui,'li the embryo at the begiu- 

 uing of the gerru band stage, showing tlie position of the caudal thiclv- 

 ening. r. th., immediately behind the blastopore, hpl., the mesoderm, 7HSd.. 

 lying just beneath the outer cell layer, erf., and the yolk cells, y. c, which 

 contribute to the formation of the endoderm. 



