142 



THE GERM-CELLS 



No less remarkable is the conjugation of spermatozoa in pairs (Fig. 68, //), which 

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

 (Ballowitz. 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. OtJier Forms of Spermatozoa 



The principal deviations from the flagellate type of spermatozoon 

 occur among the arthropods and nematodes (Fig. 69). 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. 

 69, 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 lyPoly- 

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

 {Jiihis). In the latter case the nucleus is a solid disc at the base of 

 the hat. In many decapod Crustacea the spermatozoon consists of a 



cylindrical or conical body from 



)M 



M 



^^•^ 



one end of which radiate a num- 

 ber of stiff spine-like processes. 

 The nucleus lies near the base. 

 In none of these cases has the 

 centrosome been identified. 



3. Paternal Germ-cells of 

 Plants 



In most of the flowering 

 plants the male germ-cells are 

 represented by two " generative 

 nuclei," lying at the tip of the 

 pollen tube (Fig. 106). On the 

 other hand, in the cycads (Figs. 

 87, 108) and in a large number 

 of the lower plants (pterido-l 

 phytes, Muscineas, and many 

 others), the male germ-cell is a 

 minute actively swimming cell, 

 known as the spermatozoid, which is closely analogous to the sper- 

 matozoon. The spermatozoids are in general less highly differenti- 

 ated than spermatozoa, and often show a distinct resemblance to the 



B 



Spermatozoids of C/tara. 



Fig. 70. 



JEFF.] 



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

 stage, with spermatozoids forming. C. Mature sper- 

 matozoid (the elongate nucleus black). 



