32 MARGARET MORRIS 



If the first polar nucleus fuses with the egg nucleus, cleavage 

 follows. If the second polar nucleus fuses with the egg nucleus, 

 there is no more normal development than if the second polar 

 body is formed and its chromatin rejected. It is clear that it is 

 the second maturation division, not the fii'st, that is the mass 

 reduction. 



The case of Cumingia is. then, essentially like that of Astro- 

 pecten. In both cases, the qualitative division of the matura- 

 tion is suppressed, and the polar nucleus reunites with the egg 

 nucleus. It is interesting to note that Astropecten is not a form 

 in which there is a regularly recurring parthenogenetic genera- 

 tion, but one in which development without fertilization is ab- 

 normal. Ai'temia is the only form in which parthenogenesis 

 occurs naturally where there is any indication of self-fertiliza- 

 tion. In other animals which have a regular parthenogenetic 

 generation the reducing division is not begun at all, but the egg 

 passes at once from the mass division of the chromatin to cleav- 

 age. It has already been pointed out that the eggs of Cumingia 

 in which division of the chromosomes of the first polar spindle 

 does not occur are incapable of development. Artificial par- 

 thenogenesis in this mollusc, Uke natural parthenogenesis in 

 Astropecten, is halfway between the usual conditions of arti- 

 ficial and natural parthenogenesis. In the former, development 

 is often independent of maturation; in many cases of the latter 

 the retention of the full amount of chromatin, without any 

 attempt at the qualitative division, insures development. 



In conclusion, I should hke to express my. thanks to Professor 

 Petrunkevitch for his invaluable assistance in the working out 

 of this problem. 



VIII. SUMMARY 



1. The eggs of Cumingia can be made to develop partheno- 

 genetically by exposing them to temperatures ranging from 32°- 

 37°C. and then exposing them to hypertonic sea-water. 



2. The highest percentages of cleavage-stages and swimming 

 larvae are obtained when the eggs are exposed to 32°C. for an 



