THE CHROMOSOMES IX HEREDITY. 239 



validity of the latter. The simplest form, of which chemical par- 

 thenogenesis in sea-urchins is an examble, is that in which the 

 organism has only a single chromosome series, to be represented 

 by A, B, C, D . . . N. Thus far no recognized cases of this type 

 have been reared to sexual maturity, but it is to be expected that 

 no reducing division will be found in the maturation of such 

 forms, and that their parthenogenetic offspring will exactly re- 

 semble the immediate parent. 



In cases of natural parthenogenesis which are accompanied by 

 the reentrance of the second polar body and its fusion with the 

 egg-nucleus (or its failure to form) there must be a double chro- 

 mosome series ; but we may distinguish two classes according as 

 the reducing process is accomplished in the first or the second 

 maturation division. 1 If reduction is accomplished in the first 

 division, one half the chromosomes of the oogonia are thrown 

 out and lost in the first polar body. The second division, being 

 equational, would result in a polar body which would be the 

 exact duplicate of the egg-nucleus as far as chromosomes are 

 concerned and which accordingly, by its reentrance would add 

 nothing new to the egg-series. The series after fusion would, 

 therefore, be represented by the letters A, B, C, D . . .N -{- A, B, 

 C, D . . . N. If such a type of parthenogenesis were to follow 

 sexual reproduction, the first generation of offspring might be ex- 

 pected to differ materially from the parent by reason of the cast- 

 ing out, in the first polar body, of chromosomes representing 

 certain dominant characters, and the consequent appearance in the 

 offspring of the corresponding recessives. Subsequent partheno- 

 genetic generations, however, would in each case be endowed 

 with a chromosome series exactly similar to that of the imme- 

 diate parent and accordingly might be expected to show the 

 same characters. 



In case the second division of a parthenogenetic egg were the 

 reducing division, the reentrance or suppression of the second 

 polar body would accomplish the restoration of the oogonial 

 chromosome-series. In this case the first parthenogenetic gen- 



1 Either must be regarded as possible in cases where we have no definite knowl- 

 edge since it is regularly described as the second in the Orthoptera (McClung, Sutton 

 and Copepoda (Riickert, Hacker) while in the Hemiptera-Heteroptera it is believed 

 to be the first (Paulmier, Montgomery). 



