castle: the heredity of sex. 203 



bears in these cases about half as many chromosomes as it does in 

 tlie case of the first maturation division of e<,'<j;s of the less usual 

 type. 



In the mouse, then, and perhaps in other mammals also, the first, or 

 equation, maturation division is usually, but not always, omitted ; in 

 Hydatina, however, it appears to be regularly omitted. 



C. Artemia salina. 



Weismann und Ischikawa ('88) observed the formation of only one 

 polar cell in the parth^nogenetic eggs of about a dozen different species 

 of Crustacea as well as in two species of Rotifera. Presumably their 

 observations were made exclusively on the commoner form of partheno- 

 genetic egg, the " female summer egg." In the fertilized eggs of three 

 of the same species of Crustacea (namely, Daphnia longispina, Moina 

 rectirostris, and M, paradoxa) the same authors found that tivo polar 

 cells are regularly formed. In the case of the remaining species, includ- 

 ing Artemia salina, no fertilized eggs were examined. 



Maturation of the eggs of Artemia salina has since been studied 

 by Brauer ('94) and Petrunkewitsch (: 01). Both agree that the. 

 ovarian egg contains regularly 84 chromosomes, and Petrunkewitsch 

 finds that the chromosomes are clearly double! Both observers like- 

 wise are in substantial agreement as to the method and result of 

 the first maturation division. The first polar cell and the egg con- 

 tain each 84 double (Petrunkewitscli) chromosomes. No reduction 

 division has occurred. But from this point on, the two observers differ 

 in their accounts of what happens. Petrunkewitsch stoutly maintains 

 that no second maturation division occurs; this is in accord with the 

 observations of Brauer as to a large majority of the eggs studied by 

 him, but in a certain number of eggs" he observed the occurrence of a 

 second maturation division. However, a second polar cell was in no 

 case extruded. Two nuclei wei'e formed, one peripheral, the other cen- 

 tral in ])ositioii, and these later came together and fused, exactly as male 

 and female pronuclei do in the fertilized eggs of other species, thus form- 

 ing a cleavage nucleus. Each of the two nuclei was found to contain 

 84 small chromosomes, indicating that at the second maturation division 

 a separation had taken place between the two parts of the originally 

 double chromosomes ; in other words, that the second maturation divi- 

 sion is a reduction division. Moreover, these small or part chromosomes 

 were observed to remain distinct even after the union of the two nuclei, 

 the cleavage cells containing 168 small chromosomes, whereas in eggs 



VOL. XL. — NO. 4 2 



