OTHER METHODS OF SEX-DETERMINATION 231 



duce a male. The nature of the change in the mother is 

 unknown at present.^ 



Sex-Determination through the Accidental Loss 

 of a Chromosome in Spermatogenesis. 



In hermaphroditic animals no sex-determining mecha- 

 nism has been found, and none is expected, since all the 

 individuals are alike, each with an ovary and a testis. In 



Fig. 135. 

 First and second maturation division of the sperm-cell of Angio- 

 stomum nigrovenosiim. At tlie second division (lower line) one of 

 the X-chromosomes gets caught in the division-plane. (After 

 Sehleip.) 



one species of nematode worms, Angiostomum nigro- 

 venosum, there is an hermaphroditic generation that alter- 

 nates with a generation consisting of males and females. 

 Boveri and Sehleip have shown that when the sperm-cells 

 mature in the parthenogenetic generation (Fig. 135) one 



3 In one of the worms, Dinophilus apatris, eggs of two sizes are produced 

 by each female. Both kinds give off two polar bodies, resulting in a haploid 

 pro-nucleus. Both kinds of eggs are fertilized; the larger egg produces 

 females, the smaller one males (Nachtsheim). At present the cause of the 

 production of two kinds of eggs in the ovary is entirely unknown. 



