GERM CELLS OF GRYLLOTALPA 311 



origin. Just what part they play in fertilization is not clear in 

 every case. It has been thought that the acrosome is nothing 

 more than a cap of passive metaplasmic material which may 

 aid the spermatozoon to penetrate the egg membranes. This is 

 by no means an established fact. It disappears after the entrance 

 of the spermatozoon, or, at any rate, we can no longer trace it. 

 However, it does not necessarily follow that because it becomes 

 invisible, that it has disappeared without playing any part in 

 fertilization. In the last twenty years hundreds of papers have 

 appeared on mitochondria. These bodies lie in the cytoplasm 

 and in many cases, at least, seem to arise de novo and to dis- 

 appear very much as the acrosome does. Why are we giving 

 so much attention to one and none to the other? The same is 

 true with the middle-piece. We are content to let it give rise 

 to the centrosome. My point is that in our discussion of mito- 

 chondria and chromosomes we have almost forgotten that there 

 are other structures in the cell. Some twenty-five or more years 

 ago the acrosome was relegated to the scrap heap of functionless 

 structures and there it still lies. Perhaps it should remain there, 

 but recent studies, it seems to me, have demonstrated that we 

 cannot scrutinize such structures too closely. Even those who 

 have held and still hold that the chromosomes are the actual 

 carriers of the heredity factors, are coming more and more to 

 believe that these factors are only one of a series which deter- 

 mine the character in the adult. Likewise it may be that the 

 middle-piece does more than merely carry the centrosome or 

 incite the formation of one. Sometimes the tail enters the egg 

 and sometimes it is left outside. What part, if any, it plays 

 when it enters the egg is not clear. Meves and Duesberg, how- 

 ever, maintain that, in some cases, the tail enters the egg, carry- 

 ing with it the mitochondria and thus plays an important role 

 in fertilization. 



6. CHROMOSOME NUMBERS AND SPECIES 



From his studies on the Orthoptera, McClung has maintained 

 that all the species of a closely related group should have the 

 same number of chromosomes. While this has been true of his 



