TWO KINDS OF GERM-CELLS 491 



from one ovum. Like " identical twins," the " poly-embryonic " 

 offspring are always of the same sex. In one of the armadillos 

 (Praopus or Tatusia hybrida) von Jhering found on two occasions 

 eight embryos within one chorion — presumably, therefore, from 

 one ovum — and all were males. In some of the parasitic Hy- 

 menopterous insects, e.g. Encyrtus, investigated by Marchal and 

 Bugnion, Litomastix and Ageniaspis, investigated by Silvestri, 

 one segmented ovum forms a group of embryos, all of the same 

 sex — female if the egg be fertilised, male if it be not fertilised. 

 Now, it cannot be denied that these facts strongly confirm the 

 view that the sex of the offspring is already determined in the egg. 



The theory has been more than once suggested that the 

 ova from one ovary develop into females and those from the 

 other ovary into males. Thus Dr. Rumley Dawson (The Causa- 

 tion of Sex, London, 1909) has maintained, for man, that the ova 

 produced by the right ovary develop into males, and that those 

 produced by the left ovary develop into females. This view 

 has been tested experimentally in the rat by Doncaster and 

 Marshall, who found that each rat, with one ovary completely 

 removed, produced young of both sexes. " This does not, of 

 course, prove that ' the right and left ovary hypothesis ' is 

 not true for man, but its definite disproof for another mammal 

 detracts from its probability." The theory has also been dis- 

 proved in Amphibians by H. D. King, and that it cannot apply 

 to birds is obvious, since they have only one ovary. 



Two Kinds of Spermatozoa. — In about thirty different 

 kinds of animals, such as the freshwater snail, Paludina, and the 

 freshwater beetle, Dytiscus, there are two kinds of spermatozoa 

 which differ from one another in certain details of form. It has 

 been suggested that each kind is predestined towards the develop- 

 ment of one sex, but there is no definite evidence that the dimor- 

 phism has this significance. 



The theory that in Vertebrates one testis yields male-pro- 

 ducing spermatozoa, the othsr female-producing spermatozoa. 



