MITOSIS IN GERM CELLS 117 



similar to those observed in dividing tissue-cells. For reasons 

 that will presently become apparent these nuclear divisions 

 are described as pre-meiotic.* The products of division 

 of each primitive germ-cell remain close together, so that a 

 number of cell-clusters are formed, separated from one another 

 by ingrowths of the adjacent connective-tissue. Up to this 

 period there is no distinction of sex, but about the time of the 

 metamorphosis of the tadpole into the frog, further changes 

 take place leading to the formation of ova in the female or 

 spermatozoa in the male. 



In the female frog each cell-cluster is transformed into an 

 oocyte (more rarely two or more oocytes) surrounded by a 

 sac or follicle formed by the remaining cells of the cluster. 

 This process, however, does not take place simultaneously in 

 all the clusters : only a few develop at one time, the others 

 remain in a dormant condition, and form a provision for 

 successive crops of oocytes in recurrent breeding seasons. 

 The follicular cells elaborate reserve material from the blood 

 and lymph and pass it on to the oocytes, where it is stored 

 up in the form of granules of deutoplasm or food-yolk. As a 

 result of this storage of reserve material, the oocyte increases 

 greatly in size, and at the same time its nucleus becomes larger 

 and the nuclear membrane and chromatic reticulum more 

 distinct. When it has attained a diameter of about -5 mm., a 

 thin structureless envelope, the vitelline membrane, is found 

 around it, and rather later black pigment is deposited, at first 

 over the whole surface, but afterwards it is restricted to one 

 hemisphere. By continued deposition of yolk in their cell- 

 bodies the oocytes grow till they attain the size of small shot, 

 and project in bunches from the surface of the ovary. They 

 then go through a process known as maturation, during which 

 each divides twice into two very unequal portions. At each 

 division the nucleus undergoes mitosis. The first mitosis 

 differs to a considerable degree from the normal : it was 

 formerly described as the heterotype, but more recently has 

 been called the meiotic division. The second division is 

 normal : formerly known as the homoeotype it is now called 



* I take the liberty to alter the spelling adopted by the authors of the 

 name. It is derived, they tell us, from P^UOTIKOV, a thing divided, 

 and, this being so, the English rendering should surely be " meiotic," not 

 " maiotic." 



