[ CHAMBERS-Mossop] REPORT ON CROSS FERTILIZATION 147 



lifts up from the surface of the ovum and pushes the polar bodies 

 ahead of it. In the self-fertiHzed ova the membrane forms within a 

 few minutes, the polar bodies are produced later and, therefore, come 

 to lie within the membrane. 



Development proceeded regularly in the cross-fertilized ova, but 

 more slowly than in the self-fertilized Asterias ova. Six hours after 

 mixing the sperm with the ova the Solaster sperm x Asterias ova had 

 developed into the 16-cell stage while the Asterias sperm x Asterias 

 ova had developed into the 32-cell stage and some even into later 

 stages. Except for this delay in rate and for the very much 

 decreased percentage of developing eggs, no difference was discernible 

 on comparing the living self and cross-fertilized Bipinnaria. 



To exclude the possibility that the alkalinized water alone might 

 have caused the development, Asterias ova were placed in alkaline 

 water without sperm. None developed. Also, to exclude the 

 possibility of Asterias sperm being present with the Asterias ova 

 before introducing the Solaster sperm, all the water used was carefulh' 

 heated to boiling point and then cooled. The starfish used were 

 thoroughly rinsed in such water before removing the gonads. 



We know that fertilization involves two sharply separated proc- 

 esses; first, an impetus which activates the hitherto quiescent egg 

 so that it will segment and undergo embryonic development; and 

 second, a fusion of the male pronucleus, introduced into the egg b\- 

 the spermatozoon, with the female pronucleus, a part of the original 

 nucleus of the egg. The second process involves the inheritance, in 

 the offspring, of the paternal and maternal characters. 



The first process has been independently produced in the laboratory 

 by a variety of so called "parthenogenetic" agents. The Asterias 

 ova are very easily induced to this sort of development. The appli- 

 cation of heat and mere shaking occasionally suffices to start cortical 

 changes resulting in the throwing off of a fertilization membrane 

 followed by segmentation. It is possible that the Solaster sperm 

 may have acted on the Asterias eggs only in so far as to induce them 

 to parthenogenetic development. This would explain the purely 

 maternal appearance of the larvaî resulting from the cross. Further 

 investigation of this problem is being conducted. 



Sec. IV, Sig. 11 



