Artificial Parthenogenesis in Thalassema Mellita 137 



In Fig. 60 the chromosomes are very numerous, and several divisions 

 must have occurred to produce this condition. Some of my sec- 

 tions (Fig. 61) show cases similar to those of Wilson in which 

 the chromosomes are actually found to be splitting longitudinally, 

 and there can be no doubt, when the number of chromosomes in 

 the monaster is greater than in the egg nucleus, that division of the 

 chromosomes has occurred at each active phase of the cycle. Not 

 only are the chromosomes found dividing among the rays, but more 

 frequently the longitudinal splitting takes place in the central 

 clear area of the aster, as seen in the last figure. Here one is lying 

 on the rays and is evidently in the act of dividing, while the rest, 

 grouped in the center, have either split to form double rods or are 

 taken in some stage of the process. By counting the chromosomes 

 in this and the other sections ofthe same egg, it was found that about 

 24 double rods were present, so that in all probability the division 

 which is occurring at this time is the second one in the recurring 

 transformations of the monaster. Fig. 62 shows a monaster in 

 which the rays do not quite reach the central area; it is probable 

 that the egg was killed just as the radiations were beginning to 

 reappear, 



VII GENERAL CONSIDERATIONS 

 I Dtfferetitiation Without Cell Division 



It has been shown in the preceding pages that certain solutions of 

 acids furnish an efficient stimulus for the development of unfer- 

 tilized eggs of Thalassema into embryos and larvae, and, further- 

 more, that this development in favorable experiments closely 

 approximates, if it is not identical with, the normal processes of 

 differentiation leading up to the formation ofthe swimming trocho- 

 phore. 



Previous experimenters on artificial parthenogenesis of annelids 

 have obtained with the methods which they have employed only 

 abortive attempts at development, and their embryos and larvae 

 have widely departed from the normal in almost all respects. 

 Especially aberrant is the case of Chaetopterus (Loeb, '01, Lillie 

 '02) in which the unfertilized eggs may be caused to undergo cer- 

 tain cytoplasmic differentiations in the entire absence of cell divi- 



