SEXUAL DIFFERENTIATION 



191 



Now, here we have two leading characters of the 

 sexual differentiation of gametes, a difference in size 

 and a difference in activity. Though both gametes 

 begin life as free swimming cells of identical structure, 

 one is more active than the other in the actual pro- 

 cess of conjugation. This difference in activity seems 

 here to be a mere mechanical result of the difference 

 in size. If we suppose the protoplasm of each gamete 



Fig. 23. — Conjugation of Chlamydomonas mpnadina. a. Beginning 

 of fusion of the small (male) and large (femaJe) gamete. (Note 

 that each has the normal structure of a vegetative Chlamy- 

 domonas-ceW..) b, Further stage of fusion (drawn on a larger 

 scale). Note the central area of colourless cytoplasm, derived 

 from the front ends of the two gametes, and now containing 

 the two nuclei (equal in size) in contact but not yet fused, 

 c. Spherical zygote formed within the cell wall of tiie female 

 gamete and itself clothed with a. cell wall. Note that the two 

 gamete nuclei have now fused to form the single zygote nucleus, 

 but the chloroplasts are still separate. (After Goroschankin.) 



to be attracted equally strongly towards the other, 

 the body of the small one would be more easily drawn 

 through the comparatively narrow canal formed between 

 them ; and this conclusion is supported by the unusual 

 cases figured in Fig. 24. Here the gametes have come 

 into contact obliquely, or have swung round after con- 

 tact, so that no canal is formed, btit the protoplasm of 

 both sHps out of its cell wall and the two form a zygote 



