536 



PKOCEEDINGS OF THE ACADEMY OF 



[July, 



blance. In Yoldia, Teredo and Ischnochiton, for example, the velum 

 is ultimately discarded, and a study of the diagrams (fig. 1) will 

 serve to show that the differences between the development of the 

 solenogastres and the chitons are in large measure due to the size 

 of the test or velum. Whether or not this is a fundamental fact 

 depends upon the history of the early blastomeres, which is lacking 

 at the present time; nevertheless, there are many indications that 

 the two classes of animals have descended from a common ancestor. 



Fig. 1. — Diagrams illustrating the development of the primary germ layers in 

 the chitons (A, B) and solenogastres (C, D). h, cells producing the cerebral 

 ganglia; g, archenteron; st, stomodseum; t, test cells. Such devices as stip- 

 pling, cross-hatching and parallel diagonal lines indicate homologous regions. 



The egg of Halomenia gravida is a spherical body, densely and 

 uniformly packed with yolk, and is surrounded before it leaves the 

 ovotestis with a distinct vitelline membrane. In the earliest stage 

 represented at least one polar body is distinctly visible, and imme- 

 diately beneath it the female pronucleus is clearly defined. Close 

 to the equator of the egg another nucleus, probably the male pro- 

 nucleus, appears with equal distinctness. 



