ffo.584] 



HEREDITY AND ITS MEANING 



409 



Baltzer claimed in 1909 that in the sea urchins Sphare- 

 cliinus and Echinus the sex with the dimorphic germ cells 

 is the female instead of the male, but the work of Tennent 

 has shown him to be in error and he has retracted the 

 statement. There is, therefore, no undisputed cytological 

 evidence demonstrating this type of dimorphic eggs ; but 

 since breeding results on certain species of birds and of 

 lepidopters can be interpreted only on such an assump- 

 tion, it is safe to assume that sooner or later they will be 

 found. 2 Whether or not there are animals of this type, 

 however, is of no particular importance in the present 

 discussion. What we desire to emphasize is that a large 

 number of animals, including man, have been shown to have 

 a chromatic difference between the sexes, and that this 

 difference is readily explained by the fact that the eggs 

 are of a single type and the spermatozoa of two types. 



In dioecious plants no such morphological differentia- 

 tion has been found. But this fact does not negate the 

 idea that the visible differences found in animals are really 

 sex-determining differences. We have only to suppose 

 that the dimorphism is primarily qualitative and second- 

 arily quantitative. Indeed Wilson has found that the Y 

 chromosome— the synaptic mate of the X— may vary in 

 different species from a size equal to that of X until it 

 disappears entirely, leaving X without a mate. 



There is only one criticism in this whole matter. One 

 may admit these cytological differences between the sexes, 

 but hold that they are early appearances of secondary sex- 

 ual characters. Morgan, von Baehr and Stevens have 

 answered this impeachment. In the phylloxerans and 

 aphids all the fertilized eggs produce females ; males arise 

 only by parthenogenesis, though females may arise in this 

 manner. The cytological facts are as follows: Under 

 favorable external conditions eggs develop without reduc- 

 tion and females are formed. Under unfavorable condi- 

 tions one or two chromosomes (the sex determiners) are 

 thrown out. If these eggs develop without fertilization 



~ Dimorphic eggs in Lepidoptera have recently been demonstrated by 

 both Doncaster and Seiler. 



