96 CYTOLOGY chap. 



already haploid. In such eggs, if caused to develop parthenogenetically, 

 the haploid egg nucleus acts in cleavage exactly like the zygote nucleus 

 in a fertilized egg. The resulting embryo is in consequence haploid, as 

 has been demonstrated by many observers, though originally Delage 

 erroneously supposed that the diploid number was restored by an act 

 of auto-regulation (Wilson, Toxopnetistes, igoi; Hindle, Strongylocentrotus, 

 1911, etc.). 



Eggs which do not normally mature until the spermatozoon has 

 entered vary in their reaction to the artificial developmental stimulus. 

 Some, such as the annehd Thalassema (Lefevre, 1907), on being subjected 

 to the appropriate chemical stimulus undergo maturation, throwing out 



E 



hi 







.i*' »■'•*»■ 





'^^^ 





B 



Fig. 44. 



Artificial parthenogenesis in Asterias. (Buchner, A.Z., 1911.) A, telophase of second maturation division. 

 The second polar body nucleus, instead of being thrust out of the egg, remains close to the egg nucleus. 

 B, the egg and second polar body nuclei have come into contact. Achromatic figure developing. C, the two 

 nuclei have fused. 



p.b.I., first polar body. 



two polar bodies, as if they had been properly fertilized. The haploid 

 egg nucleus then proceeds to divide as in Echinoids, and a haploid embryo 

 results. In Asterias, on the other hand (Buchner, 1911), while the first 

 maturation division is carried through normally and the first polar 

 body is cut off, the second division only proceeds normally as far as 

 telophase. Instead of the outer telophase group being extruded from 

 the surface of the egg in the second polar body, it is retained within the 

 egg and there forms a nucleus lying close to the inner group or egg nucleus 

 (Fig. 44). The egg and second polar nuclei now approach each other 

 again and fuse precisely as if the latter were the male gamete nucleus. 

 Thus the diploid number of chromosomes is restored. The resemblance 

 between this process and the rarer method of maturation of the partheno- 

 genetic egg of Artemia described by Brauer (p. 90) is striking. 



