DEVELOPMENT OF THE FROG'S EGG 



[Ch. I 



spermatozoon in two forms in which tlie process is better 

 known, and then consider the special case of the frog. 



The development of the spermatozoa of Gryllotafpa, the 

 mole-cricket, has been described by vom Rath ('92, '95). As 

 the process of spermatogenesis is relatively simple in this form, 

 and as it is, according to vom Rath, much like the process that 

 takes place in the frog, we may therefore first briefly consider 

 the changes in Gryllotalpa. 



First Period. A cell in the resting-stage at this time shows 

 a large nucleus with a distinct membrane enclosing a network 

 of fine chromatin (Fig. 1, A). The beginning of the cleavage 

 is indicated by the withdrawal of the chromatin from the 

 nuclear membrane and the thickening of the fibres of the 



A B c D 



Fig. 1. — Division of sperm-mother-cells in Gryllotalpa. (After vom Rath.) 



chromatic network. The tangled mass of threads, or net- 

 work, then takes a somewhat excentric position. This thread 

 seems to consist of linin, on which chromatin-granules are 

 arranged. Sometimes the thread can be seen to be split along 

 its length into two parts. The halves of the thread remain, 

 however, closely sticking to each other. The double thread 

 then breaks up by cross-division into twelve equal segments, 

 or chromosomes (Fig. 1, B). The chromosomes next become 

 shorter, and finally spherical, and come to lie in an equa- 

 torial plate (Fig. 1, C). When the chromatin is still in the 

 skein-stage, two minute bodies are seen in the protoplasm just 

 outside of the nuclear membrane (Fig. 1, B). These are the 

 two centrosomes, which separate more and more from each 

 other, and finally come to lie on opposite sides of the nucleus. 

 A protoplasmic spindle develops between the two centrosomes 

 (Fig. 1, C) and the fibres of the spindle become fixed to the 



