i 4 8 ARTIFICIAL PARTHENOGENESIS [CH. 



that a very small stimulus is required to induce the slight 

 cytolysis which acts as the stimulus to development. The 

 Starfish egg, unlike that of the Sea-urchin, undergoes the 

 maturation divisions after it is shed into the sea-water, and 

 as has been mentioned earlier, BUCHNER (1911) finds that if 

 the eggs are treated with carbon dioxide during the matura- 

 tion process, after the eggs are replaced in sea-water the 

 second polar division occurs, but the second polar nucleus 

 sinks back into the egg and fuses with the egg-nucleus, so 

 taking the place of a spermatozoon. 



Quite a number of Annelids have been successfully caused 

 to develop parthenogenetically, and several of them are 

 interesting from the fact that in nature the entrance of a 

 spermatozoon is necessary to bring about the maturation 

 divisions. In these it is found that treatment with the 

 parthenogenetic agent not only causes the production of a 

 fertilisation membrane, but that this is followed by the ex- 

 trusion of the polar bodies and the subsequent segmentation 

 of the egg and development into a larva. Two curious facts 

 with regard to Annelids may be noted. In Chaetopterus it 

 was found that the ordinary parthenogenetic agents induced 

 development, which went as far as a larval stage, but that 

 no true segmentation of the egg into distinct blastomeres 

 took place; segmentation, however, could be induced by 

 blood serum. And in Thalassema it was observed that the 

 polar bodies, instead of degenerating, divided mitotically 

 several times, so giving rise to groups of cells like miniature 

 embryos. 



The eggs of several Molluscs have been caused to develop 

 at least as far as early larval stages by methods of the same 

 general type as are used for Echinoderms and Annelids, but 

 in general only a small proportion of the eggs develop, no 

 fertilisation membrane is produced, and true segmentation 



