54 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



seminal vesicle and then becomes a 'gametocyte. Two individuals 

 enter into association, rounding themselves ofE^ coming into intimate 

 contact and surrounding themselves with a spherical protective cyst 

 (Fig. 21, E). Each of the two individuals so associated together is a 

 gametocyte. The nucleus of each individual undergoes repeated mitosis, 

 giving rise eventually to a great number of small nuclei which take up 

 their position in the superficial layer of the protoplasm. The latter takes 

 on an irregular lobed shape and eventually each nucleus collects a small 

 quantity of cytoplasm round itself to form a gamete. The gametocyte 



Fig. 22. 



Details of process of syngamy in Stylorhynchus (after Leger). 

 i.. Motile ( $ ) and stationary ( ? ) gametes have come in contact ; 

 2 and 3, stages of fusion ; 4, zygote. 



thus is resolved into (i) an immense number of gametes and (2) a mass 

 of residual protoplasm which is left over and disintegrates. A noteworthy 

 feature is that the gametes derived from the two gametocytes differ in 

 shape, the one lot being rounded the other pointed (Fig. 21, F), and may 

 differ very obviously also in size. There now takes place an active 

 quivering movement, the pointed gametes dashing wildly about — an 

 indication that they are provided with a flagellum as is the case with the 

 somewhat different species illustrated in Fig. 22. Syngamy now takes 

 place between pairs of gametes one pointed and one rounded (cf. Fig. 22). 

 In other words — of the two gametes which fuse together one is derived 

 from each gametocyte. In this way (Fig. 2i,fi-f^) numerous zygotes 



