io6 



THE GERM-CELLS 



takes place in the vas defer ens in the opossum (Selenka) and in some insects 

 (Ballovvitz, Auerbach). Ballowitz's researches ('95) on the double spermatozoa 

 of beetles (Dytiscidce) prove that the union is not primary, but is the result of an 

 actual conjugation of previously separate spermatozoa. Not merely two, but three 

 or more spermatozoa may thus unite to form a * spermatozeugma," which swims like 

 a single spermatozoon. Whether the spermatozoa of such a group separate before 

 fertilization is unknown ; but Ballowitz has found the groups, after copulation, in 

 the female receptaculum, and he believes that they may enter the egg in this form. 

 The physiological meaning of the process is unknown. 



2. Other Forms of Spermatozoa 



The principal deviations from the flagellate type of spermatozoon 

 occur among the arthropods and nematodes (Fig. 51). In many of 

 these forms the spermatozoa have no flagellum, and in some cases they 

 are actively amoeboid; for example, in the daphnid Polyphemus (Fig. 

 51, A, B, C) as described by Leydig and Zacharias. More commonly 

 they are motionless like the ovum. In the chilognathous myriapods 

 the spermatozoon has sometimes the form of a bi-convex lens (Poly- 

 desmus), sometimes the form of a hat or helmet having a double brim 

 (Julus). In the latter case the nucleus is a solid disc at the base of 

 In many decapod Crustacea the spermatozoon consists 



of a cylindrical or conical body 

 from one end of which radiate 

 a number of stiff spine-like 

 processes. The nucleus lies 

 near the base. In none of 

 these cases has the centrosome 

 been identified. 



the hat. 



Fig. 52. Sperm atozoids of Chara. [BELAJEFF.] 

 A. Mother-cells with reticular nuclei. D. Later 



stage, with spermatozoids forming. C. Mature sper- 



matozoid (the elongate nucleus black). 



3. Paternal Germ-cells of 

 Plants 



In the flowering plants the 

 male germ-cell is represented 

 by a ''generative nucleus," to- 

 gether with two centrosomes 

 and a small amount of cyto- 

 plasm, lying at the tip of the 

 pollen-tube (Fig. 80, A). On 



the other hand, in a large number of the lower plants (Pteridophytes, 

 Muscineae, and many others), the male germ-cell is a minute actively 

 swimming cell, known as the spermatozoid, which is closely analogous 



