EFFECTS OF MIXING DIVERSE ORGANISMS 267 



organisms, the development of the egg produced offspring 

 that were not intermediate between the two parental types, 

 but were like the mother only. Sea urchin ova fertilized by 

 starfish, holothurians, mollusks or annelids, gave rise to 

 typical sea urchin young, showing no influence of the male 

 parent. 



What is the cause for this result? Why are the chromo- 

 somes and genes of the sperm without effect on the charac- 

 teristics of the offspring? 



A careful cytological examination by Kupelwieser 

 (1909) ^ of developing sea urchin eggs that had been fer- 

 tilized by mollusk sperm showed that the chromosomes of 

 the sperm do not enlarge, as is usual after fertilization. 

 They do not unite with the chromosomes of the ovum, do 

 not divide, take no part in development, and ultimately 

 die in the early stages of development (figure 52, A). This 

 then is the explanation of the fact that the offspring of the 

 cross show none of the characteristics of the male parent. 

 The paternal chromosomes play no role in development 

 and therefore do not affect the characteristics. What the 

 sperms do is merely to initiate development; for the rest 

 development is essentially parthenogenesis; it is develop- 

 ment of the ovum alone. 



In such cases the sperm are unable to live and develop 

 in the cytoplasm of a very diverse organism; there is in- 

 compatibility between the two. 



In other crosses between very diverse types, essentially 

 the same thing occurs, although there are differences in de- 

 tails. When the sea urchin ovum is fertilized by annelid 

 sperm, the sperm nucleus unites in the usual way with the 

 nucleus of the ovum, so that the two form a single nucleus. 

 But before this nucleus divides, it ejects the annelid chromo- 

 somes, which are then lost in the cytoplasm (figure 52, B). 

 The egg continues development with only the sea urchin 

 chromosomes, producing only sea urchin young. 



