KEPRODTJCTION BY GERM-CELLS 



379 



which take the form of sperm-threads or spermatozoa, and yet they 

 diSer very much from one another in detail. It would undoubtedly 

 be of great interest to follow out these minute adaptations of the 

 sperm-cells to the conditions of fertilization, and to demonstrate that 

 their size, and especially their form, in the different species of animals 

 are adjusted to the special constitution of the ovum, its envelope, 

 and its micropyles, and to the ease or difficulty with which it can be 

 reached; but much information must be forthcoming 

 before we can even suggest, for instance, why the sperm- 

 cell of the salamander is so enormously long, large, and 

 pointed at the head, while that of Man (Fig. 67, i) is 

 comparatively short, with broad, flat head and a recently 

 discovered minute apex ; or why those of Man and many 

 fishes (such as Cobitis) should be so much alike, and so 

 on. From many sides, however, we are led to conclude 

 that even down to the minutest details nothing is in 

 vain, and that everything depends on adaptation. 



In general, even the peculiarities of form already 

 indicate this; thus the spirally coiled structure of the 

 head, which is especially well developed in the sperma- 

 tozoa of birds (Fig. 67, 5), in those of the skate (7), 

 and of the freshwater snail (Paludina) (10), works like 

 a corkscrew, and makes it possible for the sperma- 

 tozoon to pierce through the resistant envelope of the 

 ovum. Similarly, the sharply pointed head of the insect 

 spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping 

 through the minute pre-formed micropyle in the hard 

 egg-shell. 



Of the detailed and complicated structure of 

 spermatozoa we have only recently been made aware 

 through the increasing perfection of the microscope and 

 of technical methods of investigation. Fig. 68 shows 

 one after a diagrammatic figure by Wilson. We see the 

 apical point (sp) for boring into the ovum, the nucleus 

 (n) surrounded by a thin layer of protoplasm, which 

 together form the head, then the middle portion (m) which contains 

 the ' centrosome ' (c), and the ' tail ' or ' flagellum ' which effects the 

 movement of the whole and which itself possesses a complex structure 

 with an ' axial filament ' {ax) and an enveloping layer, the latter often 

 drawn out into a spirally twisted, undulating membrane of the most 

 extreme delicacy, as is most clearly seen in the newt (Fig. 67, 6). 



Not in the Daphnids alone, but in other groups of Crustaceans as 



Fio. 68. 



Diagram of a 



spermato- 

 zoon, after 



Wilson. 



sp, apical 

 point. 



n, nucleus. 



c, centrospere. 



m, middle 

 piece. 



ax, axial fila- 

 ment. 



e, terminal 

 filament. 



