REPRODUCTION BY GERM-CELLS 375 



itself like a screw through a narrow opening in the egg-envelope, 

 whether the opening was previously present or not. 



We can thus understand why, among insects for instance, the 

 male cells should always occur in the form of zoosperms, although 

 in this case they reach a special receptacle in the female reproductive 

 organs, the ' receptaculum seminis,' and are stored up in this. When 

 a mature ovum gliding downwards through the oviduct comes to 

 the place where this receptacle opens into it, the liberation of a few 

 sperm-cells suffices to fertilize it with certainty, provided that they 

 possess the thread-like form, which allows them to slip in through 

 the very minute opening in the egg-envelope. It might be inferred 

 from the certainty with which the ovum must in this case be found 

 by the spermatozoon that only a small number of the latter would 

 require to be produced, and yet even here the number is very large, 

 though not so enormous as in the sea-urchins and other marine 

 animals, which simply allow the sperm-cells to escape into the water. 

 The large number in insects is due to the fact that many of the 

 sperms may miss the micropyle ; and also that in many insects a 

 very large number of eggs have to be fertilized in succession. In the 

 course of a life lasting three or four years the queen bee lays many 

 thousand of eggs, most of which are fertilized, and that from a 

 seminal receptacle which has been filled only once. 



There are, however, other sperm-cells of thread-like form which 

 are not produced in such enormous multitudes, but in a much more 

 moderate number, perhaps a few hundreds in the testicle. This is so 

 in the little Crustaceans, known as Ostracods, all the freshwater species 

 of which possess zoosperms only moderately numerous and of quite 

 unusual size. 



The comparatively small number is explained by the certainty 

 with which each of them reaches the ovum, and the large size may be 

 accounted for in part by the small number which suffices, and which, 

 therefore, admits of the male cell also carrying a considerable portion 

 of the material for the building up of the embryo. Probably, how- 

 ever, the thickness and firmness of the covering of the ovum has some- 

 thing to do with it, for it has no opening for the entrance of the 

 male cell, and it is fully hardened by the time fertilization takes 

 place. Perhaps nowhere can we see more clearly how every little 

 detail of the structure of the organism is dominated by the principle 

 of adaptation than in the arrangements for fertilization, and notably 

 in those which obtain in the Ostracods. I pass by the complicated 

 apparatus for copulation, since we do not yet understand it fully in 

 all particulars. According to my own investigations and those of my 



S 2 



