SEX-CHROMOSOMES AND INHERITANOE 185 



the two larger cells each contain one X-chromosome and 

 two autosomes. They correspond obviously to the female- 

 producing sperm of other insects. Hence only females 

 arise from fertilized eggs. 



The rotifers, especially Hydatina senta, are the only 

 animals in which the transition from parthenogenetic to 

 sexual reproduction has so far been gotten under con- 

 trol by regulating the environment, and although the 

 evidence that the environment causes part of its effects by 

 influencing the chromosomal mechanism is not yet estab- 

 lished, there is, in my opinion, some indication that such 

 is the case. The common method of reproduction in 

 Hydatina is as follows: A parthenogenetic female (Fig. 

 85, A) lays eggs (D), each of which, after giving off a 

 single polar body, develops at once {i.e., without fertiliza- 

 tion) into a female like the mother. The whole number 

 of chromosomes is retained in the eggs. Several or many 

 generations may be produced in this way. Whitney has 

 shown that if such females are fed on a green alga, 

 Euglena, daughters appear ( structurally like the others) 

 that produce smaller eggs {E). If these eggs develop 

 without fertilization they become males (C). Examina- 

 tion of these small eggs show that they give off two polar 

 bodies, and retain a reduced number of chromosomes. This 

 process is the same by which the male bee is produced. 



If the female, that produces the small eggs just 

 described from which the males develop, should have been 

 impregnated by a male soon after she hatched, her eggs 

 would then grow larger and surround themselves with a 

 thick-walled coat. They become the winter or resting 

 eggs. Each such Qgg, after the sperm enters, gives off two 

 polar bodies, reducing in this way the number of its chro- 

 mosomes. By the addition of the sperm nucleus the full 

 number of chromosomes is recovered. 



Whitney has recently shown that there are two classes 

 of spermatozoa produced by the male, large and small; 



