SEX DETERMINATION 253 



influence on the male has been noted, nor is the sex of the 

 female changed but merely her sexual development repressed. 



An interesting case of sex control through secretions has 

 recently been discovered in a mollusk, Crepidula. The in- 

 dividuals of this species normally function as males when 

 they are small, at that time developing sperm, but when 

 grown to larger size they develop eggs and function as fe- 

 males. Since eggs and sperm are not developed simultane- 

 ously in the same individual, the eggs are regularly cross 

 fertilized. Gould has shown that if a small Crepidula is iso- 

 lated from other individuals it remains a "neuter," but that 

 if it is brought within a few millimeters of a large (female) 

 individual, it proceeds to develop as a male and liberates 

 sperm. The action is supposed to result from substances 

 given off into the sea water from the body of the nearby 

 female. As the individuals of Crepidula remain in one place 

 practically throughout their adult life, this curious adapta- 

 tion has manifest advantages to the species. 



The egg or larger gamete (the so-called macro-gamete) in 

 all animals is non-motile and contains a relatively large 

 amount of reserve food material for the maintenance of the 

 developing embryo. This reserve food material it is the func- 

 tion of the mother to supply. In the case of some animals, 

 for example flatworms and mollusks, the food supply of the 

 embryo is not stored in the egg-cell itself, but in other cells 

 associated with it, which break down and supply nourish- 

 ment to the developing embryo derived from the fertilized 

 egg. Again, as in the mammals, the embryo may derive its 

 nourishment largely from the maternal tissues, the embryo 

 remaining like a parasite within the maternal body during 

 its growth, feeding by osmosis. But in all cases alike the 

 mother supplies the larger gamete and the food material 

 necessary to carry the zygote through its embryonic stages. 

 The father, on the other hand, furnishes the bare hereditary 

 equipment of a gamete, with the motor apparatus necessary 

 to bring it into contact with the egg-cell, but without food for 

 the developing embryo produced by fertilization. The ga- 



