GERM CELLS OF COELENTERATES 607 



parison is not so satisfactory since the point of view of the 

 chromosomes would make a greater difference in apparent shape 

 than in size. However, the comparison of shape may be made 

 in the same manner as that of size and the results will be the 

 same. 



Because of the apparent lack of constancy in the size and 

 shape of chromosomes, it hardly seems possible that there is a 

 reappearance of the same chromosome in succeeding generations. 

 Such evidence as we get does not warrant the conclusion that 

 the chromosomes are the same in size and shape in every cell, 

 generation after generation. The evidence does not, perhaps, 

 permit us to conclude that the chromosomes of succeeding gen- 

 erations are distinctly different in size and shape, but it seems 

 best to remain somewhat skeptical with regard to the genetic 

 relationship of chromosomes till the evidence demonstrates it 

 beyond question. 



It may be stated that although the material here used, the 

 medusa Aglantha, is far more satisfactory for a study of chromo- 

 somal behavior than any other coelenterate known to me, it 

 may still not be so favorable as material obtained in other phyla. 



And yet it is certain that this material is far more favorable 

 than that of some other animals. This would be obvious to one 

 who has seen this material and that from other sources. In a 

 cytology course given to students I have used the oogenesis and 

 spermatogenesis of insects, Crustacea and vertebrates as mate- 

 rial for study; from a knowledge of the character of the chromo- 

 somes of these forms I have no hesitation in saying that the 

 chromosomes of Aglantha are as satisfactory for a study of 

 chromosome behavior as most of the groups mentioned. 



CONCLUSIONS 



The germ cells of Aglantha are formed by a proliferation of 

 the ectoderm cells of the sub-umbrella in the aboral region of 

 the bell. The division of these cells continues for a time, always 

 by mitosis, until a large number are present in the gonad when 

 the division period ends, at about the same time in all the gonads 

 of the individual. The nucleus of an oogonium is large and the 

 nuclear contents so like the cytoplasm in appearance as to be 



