EFFECT OF RADIUM ON DEVELOPMENT OF CtLETOPTERUS. 53 



The unfertilized eggs were exposed for periods varying from 

 5 to 135 minutes, after which they were inseminated with 

 Ch&topterus sperm, and their behavior in the formation of polar 

 bodies and in cleavage recorded. In all of these experiments 

 controls were kept, made up from the same lot of eggs and subject 

 to the same temperature conditions. 



For cytological study the radiated eggs were killed, at various 

 times after insemination, in Meves' fluid, Boveri's picro-acetic, 

 and in Bouin's fluid. The last named reagent proved to be the 

 best for most purposes. It acts rather destructively on the cen- 

 trospheres, but is otherwise very good. 



THE NORMAL DEVELOPMENT OF Chcetopterus. 



The process of maturation and fertilization in Chcetopterus has 

 been described by Mead ('97). When the eggs are taken from 

 the genital segments of the female and put into sea water the 

 wall of the germinal vesicle breaks down. Fibers from two well- 

 developed egg asters grow in toward the chromosomes which 

 gradually assume an equatorial position. The spindle thus 

 established moves toward the animal pole, rotating as it goes, 

 until it comes to rest at right angles to the periphery of the egg. 

 The chromosomes are already split, being tetrads at this time, 

 but no actual separation occurs until after the sperm has pene- 

 trated the egg. In this condition the egg remains until fertilized. 



As soon as the sperm has penetrated the egg the first polar body 

 is quickly extruded, about 15 minutes after insemination. The 

 second polar body is given off about 10 minutes later. Each 

 chromosome remaining within the egg bends into a V shape and 

 the entire group of nine gathers about the remaining egg aster. 

 Before the group begins to move inward toward the center of the 

 egg each chromosome becomes vesicular. During the inward 

 journey the separate vesicles fuse to form a single large egg nu- 

 cleus. The egg aster disappears when the fusion of the chromo- 

 somal vesicles takes place. 



The sperm head does not begin to grow until it has penetrated 

 the egg for some distance. But before its growth commences a 

 small sperm aster with a centrosome appears in front of it. As 

 the sperm moves inward the centrosome divides, each daughter 



