EXPLANATION OF PLATE VL 



s. c. segmentation cavity. 



es. embryo-swelling. 



7ng. medullary groove. 



fs. tail-swelling. 



h. head. 



ch. notochord. 



op. eye. 



au. r. auditoiy vesicle. 



al. alimentary canity. 



so. s. somatic stalk. 



I. r. r. 1st visceral cleft. 



re. visceral cleft. 



7np. muscle-plate or protover- 

 tebrae. 



m. mouth. 



an. point where anus will appear. 



al. v. alimentary vesicle at the pos- 

 terior end of the alimentary 

 canal. 



lit. heart. 



A. Surface view of blastoderm of Pristiurus hardened in chromic acid. 



B. Surface view of fresh blastoderm of Pristiurus. 



C. D, E, and F. Pristiurus embryos hardened in chromic acid. 

 G. Torpedo embryo viewed as a transparent object. 



H, 1. Pristiurus embryos viewed as transparent objects. 



