326 N. M. Stevens. 



portion of the cycle. The whole subject needs further investiga- 

 tion from the standpoint of the determination of sex. 



3. Determination of Sex. 



As the male sex cells of the Aphid contain no "accessory chro- 

 mosome," McClung's theory of sex-determination need not be 

 discussed in this connection. The question naturally arises 

 whether the "accessory chromosome" is to be found in any of the 

 parthenogenetic insects or crustaceans. 



Castle ('03) in his recent paper on "The Heredity of Sex" 

 attempts to place the sex-character in the same category with other 

 hereditary chracters and to apply to it the principles of Mendel's 

 Law of Heredity — dominance and segregation. He says on 

 page 198, "A study of sex-heredity in parthenogenetic animals 

 shows (i) that in such animals the female character uniformly 

 dominates over the male whenever the two are present together," 

 and on page 199, "With a single exception, we know that in unin- 

 terrupted parthenogenetic reproduction, as it occurs in the 

 Daphnidae and Rotiferae at certain seasons, the parthenogenetic 

 egg forms only one polar cell, and the animal developing from 

 such an egg is invariably female, or more correctly d^ (9 ), the male 

 character being recessive," and further on, "At the return to 

 sexual reproduction, the parthenogenetic mother produces eggs 

 which form a second polar cell, and from such eggs only males 

 develop. It is clear, then, that in the second maturation division 

 the female character has been eliminated from the egg, for were it 

 still there, it must from its nature dominate." 



As an exception Castle cites Rhodites rosae, in which according 

 to Henking the parthenogenetic eggs produce two polar bodies, 

 but no reduction occurs, and therefore no segregation of sex 

 characters. Castle assumes that the occasional egg which pro- 

 duces a male, does, in some way, eliminate the female character. 

 Hydatina senta is also cited: according to Lenssen ('98) no polar 

 body is formed and there is no reduction in the female egg, while 

 in the male egg one maturation division occurs with reduction. 

 Castle supposes in the latter case that the first maturation division 

 is regularly suppressed as Sobotta ('99) has found to be the case in 

 the mouse. 



In the Aphid only one polar body is formed in the female and 



