CASTLE: THE HEREDITY OF SEX. 203 
bears in these cases about half as many chromosomes as it does in 
the case of the first maturation division of eggs 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 regular/y omitted. 
C. ARTEMIA SALINA. 
Weismann und Ischikawa (’88) observed the formation of only one 
polar cell in the parthenogenetic 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 ¢wo 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 dowble/ 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 (Petrunkewitsch) 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 secoud polar cell was in no 
case extruded. ‘Two nuclei were formed, one peripheral, the other cen- 
tral in position, 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 
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