452 



WILSON. 



[Vol. VI. 



mollusks and annelids. This reduction stands in obvious rela- 

 tion to the different size of the cells produced at the two poles, 

 and requires no further discussion. 



Diagram VIII. 



A. Four-celled stage of true radial type (^Amphioxus, Synapta). 



B. Spiral type (^Nereis, Crepidula). 



C. Side-view of the last. 



D. Spiral type {Discoccelis). 



E. Eight-celled stage (^Discoccelis). 



