30 MARGARET MORRIS 



ing to normal development. Although Hertwig's series of stages 

 is not complete, it is more nearly so than Brauer's. 



Up the present time there has never been any definite proof 

 that in artificial parthenogenesis development depended on the 

 I'etention of polar bodies or the restoration in some way of the 

 full number of chromosomes. In the eggs of the sea-urchin, 

 which are among the most favorable objects for parthenogenetic 

 study, fairly normal plutei are obtained from eggs which contain 

 the haploid number of chromosomes. Here the reduction divi- 

 sions are completed before the egg is subjected to parthenoge- 

 netic treatment, and there is no later regulation of the number 

 of chromosomes. 



Some experimental studies have been made of eggs which are 

 laid before the maturation divisions begin, with the object of 

 finding out whether they would respond to treatment better in 

 this stage than after the polar bodies have been formed. In 

 working with the starfish, Asterias glaciahs, Delage ('01) obtained 

 the best results by applying the treatment during the anaphase 

 of the first polar spindle. From this and from the fact that the 

 second polar body is not formed in the majority of the eggs that 

 develop, he concluded that the essential factor was the reten- 

 tion of chromatin. Garbowski ('03) also worked on Asterias 

 glacialis, but according to his results, eggs with two polar bodies 

 develop rather better than those with only one or none at all, 

 though all three kinds may develop. Later, Delage ('04) modi- 

 fied his idea, stating that the reason the stimulus must be applied 

 during the maturation period was that the egg responds more 

 readily when it is in the process of karyokinetic division than 

 in the resting stage. 



In many other cases of artificial parthenogenesis, it seems to 

 make no difference whether the polar bodies are formed or not. 

 In Thalassema, for instance, (Lefevre, '07) eggs which have 

 formed polar bodies develop as well as those wh'ch have not. 

 In this form the first polar division may be suppressed, the 

 spindle giving rise to two nuclei which fuse to form a cleavage 

 nucleus. There is evidence that a second division of each of the 

 two nuclei may take place before they fuse, giving a stage with 



