108 THE GERM-CELLS 



and mosses. In the ferns and other pteridophytes a somewhat dif- 

 ferent type occurs (Fig. 53). Here the spermatozoid is twisted into 

 a conical spiral and bears numerous cilia attached along the upper 

 turns of the spire. The nucleus occupies the lower turns, and 

 attached to them is a large spheroidal cytoplasmic mass, which may, 

 however, be cast off when the spermatozoid is set free or at the time 

 it enters the archegonium. This, according to Strasburger, proba- 

 bly corresponds to the basal cytoplasmic mass of Chara. The upper 

 portion of the spire to which the cilia are attached is composed of 

 cytoplasm alone, as in Chara. 



The homologies, or rather analogies, between the respective parts 

 of the spermatozoid and spermatozoon are not yet very definitely 

 established, since the history of the spermatozoid in fertilization has 

 not yet been accurately followed. Strasburger ('92) believes that the 

 anterior cytoplasmic region, to which the cilia are attached, consists 

 of " kinoplasm " (archoplasm), and hence corresponds with the mid- 

 dle-piece of the spermatozoon. If this view be correct, there is, on 

 the whole, a rather close correspondence between spermatozoid and 

 spermatozoon, the flagella being attached in both cases to that end of 

 the cell which contains the centrosome or kinetic centre, the nucleus 

 lying in the middle, while the opposite end consists of cytoplasm (i.e. 

 the apex of the spermatozoon, the cytoplasmic vesicle of pterido- 

 phytes, the basal cytoplasm of Chara, etc.). The attachment of the 

 flagella in both cases to the archoplasmic region is a significant fact, 

 for Strasburger believes that they arise from the " kinoplasm " (archo- 

 plasm), and it is probable that the spermatozoon tail has a similar 

 origin (p. 126). 



C. ORIGIN AND GROWTH OF THE GERM-CELLS 



Both ova and spermatozoa take their origin from cells known as 

 primordial germ-cells, which become clearly distinguishable from the 

 somatic cells at an early period of development, and are at first exactly 

 alike in the two sexes. What determines their subsequent sexual 

 differentiation is unknown save in a few special cases. From such 

 data as we possess, there is very strong reason to believe that, with 

 a few exceptions, the primordial germ-cells are sexually indifferent, 

 i.e. neither male nor female, and that their transformation into ova 

 or spermatozoa is not due to an inherent predisposition, but is a reac- 

 tion to external stimulus. The nature of the stimulus appears to 

 vary in different cases. Thus Maupas's experiments seem to show 

 conclusively that, in rotifers, the differentiation may depend on 

 temperature, a high temperature tending to produce males, a low 





