THE FROG : HISTOLOGY, GERM CELLS, DEATH 103 



Nuclear 

 Division. 



in a muscle fibre conduction is relatively poorly developed 

 and contraction powerful. In both these tissues chemical 

 manufacture and secretion for the benefit of the rest of 

 the body is at a low ebb, while in gland cells, which are 

 neither contractile nor conducting, it is highly developed. 

 In correspondence with these peculiarities of function go 

 peculiarities of form. That is to say, here as everywhere 

 we find differentiation and the physiological division of 

 labour hand in hand. 



The innumerable nucei of the frog's body have all arisen 

 by the di- 

 vision of 

 one origi- 

 nal nucleus, that of 

 the zygote formed by 

 the conjugation of the 

 ovum and spermato- 

 zoon (seep. 10). The 

 process of nuclear 

 division by which the 

 nuclei multiply is 

 usually followed by a 

 cell division in which 

 each half of the 

 divided nucleus takes 

 its own portion of the 

 cytoplasm which sur- 

 rounded the parent 

 nucleus, but in some 

 cases, as in the di- 

 vision of the nuclei 

 of a striped muscle 

 fibre, cell division 

 does not take place, so that a coenocyte arises. Nuclear 

 division is of two kinds. In a few cases, as in some 

 leucocytes, the process is quite simple. The nucleus 

 lengthens, then narrows in the middle so that it becomes 

 dumb-bell shaped, and finally breaks into two at the narrow 

 part. This is simple or amitotic division. In most cases, 

 however, a complicated process known as karyokinesis 

 or mitosis takes place. The resting nucleus is enclosed 



f-e- 



Fig. 62. — Part of one of the fat bodies of a 

 frog, compressed and magnified, show- 

 ing fat cells with fat globules in various 

 stages. 



/■§;■■, Fat globules ; ««., nuclei. 



