GERM CELLS OF COELENTERATES 623 



embryonic cells can be found. The differentiated cells must 

 undergo a regressive change and themselves become the em- 

 bryonic cells both in appearance and potency. If all the cells 

 which are embryonic may be considered as germ cells the cells 

 from which they arise are entitled to the same name; but the 

 original cells are differentiated body cells, and this means body 

 cells may give rise to germ cells. This is a direct contradiction 

 of the germ plasm theory. 



GROWTH OF OOCYTES 



In mature medusae the germ cells extend in a swollen mass 

 about the middle of the stomach, forming a well marked gonad. 

 The cells within this gonad are numerous, closely packed to- 

 gether, and are alike in size and structure in the early stages. 

 In the ovary only a few of these cells grow to produce mature 

 egg cells, the ones which get the start are the ones which grow, 

 the others remain unchanged and serve as food for the growing 

 cells. The oocytes which fill the ovary in a gonad in which 

 no growth has occurred have the structure and appearance indi- 

 cated in figure 51. In the ovary from which these cells were 

 taken there was no large ovum and only a single oocyte had 

 begun to grow (fig. 52). The oocytes have a deeply staining 

 compact cytoplasm with a fairly large nucleus, faintly staining, 

 and a large deeply staining nucleolus. The latter with iron- 

 hematoxylin stains an intense black; with hematoxylin and 

 eosin the acid stain is the one which stains the nucleolus, though 

 the hematoxylin may stain the nucleolus slightly along with the 

 eosin. There may be a few vacuoles in the nucleolus at this 

 time but as a rule it appears homogeneous. The rest of the 

 nucleus stains very faintly in iron-hematoxylin and a little more 

 with Delafield's or Ehrlich's hematoxylin. There is a fairly even 

 distribution of this nucleoplasm, with a very faint hint of a deli- 

 cate reticulum in a few cases. 



In oocytes of this age, that is before growth, there is often 

 an accumulation of more deeply staining material in the nucleus 

 arranged in a ring about the nucleolus (fig. 51). In every oocyte 

 there is a more or less rich accumulation of deeply staining 



