GERM CELLS OF COELENTERATES 597 



and growth was just beginning in a few of the oocytes. The 

 cells closer to the cavity of the gonad are oogonia, those in a 

 zone toward the outside are oocytes; of the latter there are two 

 in the figure which have grown to a considerable size. This 

 arrangement of oogonia and oocytes in zones is characteristic 

 of young gonads but is not observed after growth has begun. 



The oogonia possess nuclei which occupy the greater portion 

 of the cells, leaving only a narrow border of cytoplasm. The 

 cytoplasm and the nucleus of such a cell are filled with an evenly 

 distributed, finely granular, and close packed protoplasm which 

 shows no particular structure. It makes no difference what 

 fixation is used, the cytoplasm and nucleus always have this 

 appearance and are so similar it is possible only with high powers 

 of the microscope to clearly define the boundary line, and only 

 with an oil immersion can a nuclear membrane be demonstrated. 

 Within the nucleus is a very prominent, deeply staining nucleolus 

 which appears to be largely chromatic in composition and may 

 contain the greater portion of the chromatin of the nucleus. 

 Figure 2 is an oogonium as seen under high magnification, with 

 the nucleus represented as being a little too sharply defined. 

 The impression one gets from a superficial examination of an 

 ovary of this age (as figure 1 suggests fairly well) is that of cells 

 with very small, sharply defined, deeply staining nuclei, which 

 appear too small for germ cells. Close observation with high 

 powers makes it clear that this body is a nucleolus which is 

 within a large and ill-defined nucleus. The same appearance 

 is characteristic of spermatogonia. 



According to our knowledge of the behavior of germ cells in 

 many animals we should expect to find the contraction phase 

 (synizesis) of the chromatin, and the synapsis or conjugation 

 stage of the germ cells occurring after the last oogonial division 

 before, or just as, growth starts. There is no difficulty in deter- 

 mining which cells are oogonia and which oocytes after growth 

 has started, and even in the younger gonads a difference in size 

 is often observed. But even in the absence of marked size 

 differentiation the different stages may be recognized by posi- 

 tion, since the oogonia are grouped near the cavity of the gonad 



