GERM CELLS OF COELENTERATES 613 



GERM CELLS IN PRIMARY MEDUSAE 



The germ cells located in the ectoderm of the stomach wall 

 produce a gonad in the form of a thickened ring about that 

 organ (fig. 38). It is not certain that the germ cells take their 

 origin from the ectoderm (critical early stages were missing) 

 but the appearance of the gonad and other tissues of the stom- 

 ach suggests such an origin. It may be possible that germ cells 

 are present in the wall of the hydroid polyp from which the 

 primary medusae arise, but since the hydroids are so rarely, if 

 ever, seen such a determination can not soon be made. I have 

 seen a single specimen of the hydroid which had been preserved 

 for many years in alcohol and was of no service for cytological 

 study, even if it were not more valuable as a part of a collection 

 of hydroids. It hardly seems probable that the germ cells are 

 present in the polyp since sections of medusae show many indi- 

 viduals in which no such cells are recognizable. The lack of 

 germ cells suggests either an old individual which has shed all 

 its sexual products, or an immature individual in which germ 

 cells are not yet differentiated. Size should be something of a 

 criterion in determining which alternative to accept, and since 

 some of the medusae without recognizable germ cells are smaller 

 than the average we may conclude that these, at any rate, are 

 immature. 



When germ cells are present, even if only a few in number, 

 they are easily recognized by their position, size, shape, com- 

 pact, deeply staining cytoplasm and usually by a large nucleus. 

 On the right side of the stomach wall in figure 39 a portion of 

 the ovary is represented in the swollen mass which is composed 

 of a group of small oocytes. In no case has the gonad been 

 observed to extend further aborally than here shown; there is 

 always a space between the top of the gonad and the place where 

 the sub-umbrella meets the stomach. The same figure (39) 

 shows an actinula or larval polyp in position on the left side of 

 the stomach. This has developed in this position and is about 

 ready to be liberated. Attention is also directed to the swollen 

 base and a portion of the tentacle at the lower right corner of 

 the figure; it is from this bulb that secondary medusae are pro- 

 duced by budding. 



