A Cytological Study of Artificial Parthenogenesis etc. 147 



BOVERI ('02) and PETRUNKEWITSCH ('04) have both shown that 

 the number of chromosomes normally occurring in the cells of this 

 species is 36, and that the number occurring in the unfertilized egg 

 is 18. The latter is the number of chromosomes found by DELAGE in 

 the parthenogenetic larvae, and therefore his own observations support 

 the idea that after chemical fertilization the full number of chromo- 

 somes is not restored during subsequent development. In spite of 

 this, however, in a recent paper DELAGE ('08) still adheres to his 

 former view that the number of chromosomes constant for the species 

 is restored in the cells of the parthenogenetic larvae. This view 

 seems to be based mainly on theoretical considerations for no ob- 

 servations are brought forward in support of it 1 ). 



WASSILIEFF ('02) and PETRUNKEWITSCH ('04) have studied the 

 cytological effects of artificial fertilization in echinoids, but in both 

 cases their methods and results were imperfect, as they used the 

 older method of treatment with a hypertonic salt solution in order 

 to cause development. 



The only complete account of the cytological details of artificial 

 parthenogenesis in echinoids is WILSON'S ('01), for the later studies 

 of HERBST ('07, '09) and other authors, do not concern the finer 

 cytology of development. As GODLEWSKI ('09) has given a summary 

 of this subject it will be unnecessary to discuss in detail the various 

 studies of artificial parthenogenesis in other groups that have ap- 

 peared during the last few years. In all cases where the number 

 of chromosomes in the parthenogenetically developing eggs was deter- 

 mined the reduced number was found to occur, but these observa- 

 tions have been confined to the first few divisions. On these grounds, 

 therefore, DELAGE considers the question undecided, as the restoration 

 to the full number is supposed to take place gradually. 



In the following account it will be seen that the reduced number 

 of chromosomes persists in the cells of parthenogenetic embryos of 

 Strongylocentrotus purpuratus as far as the free swimming blastula, 

 and beyond this stage it is impossible to count them owing to the 

 small size of the cells. These observations, therefore, agree with 

 those of KOSTANECKI ('04, '08) on Mactra, LEFEVRE ('07) on Tha- 



*) >0n voit que, si les partisans de la non-regulation du nombre des chromo- 

 somes avaient raison, leur nombre devait se reduire de moitie" a chaque gene- 

 ration, jusqu'a ce que la reduction devienne impossible en presence d'un nombre 

 impair ou, finalement, de 1'unite* (DELAGE, p. 497). The logic of this remark is 

 not very forcible. 



10* 



