292 A. FRANKLIN SHULL 



represent more than thirteen cells, and one of them mcludes only 

 five. In the absence of any knowledge of the variability of the 

 cells of the intestine, of the variability of the individual animals 

 reared under like conditions, and of the precautions taken to 

 prevent unconscious selection in making the drawings, no cri- 

 terion of the value of the above figures is possible. At least it 

 cannot be said that Papanicolau's conclusions are incorrect. 



Hartmann ('19 a) studied other species of Cladocera, employing 

 the hypodermis and certain ganglia as well as the intestine, and in 

 general confirms Papanicolau's conclusions. His observations on 

 changes associated with differences of temperature and differences 

 of position in the cycle appear to have been made on animals 

 collected from nature, rather than on experimental animals, so 

 that one may question what other differences in the environment 

 were involved; but his conclusion is that low temperature (also 

 Hartmann, '19 b) and long-continued parthenogenesis increase 

 the ratio K/P. An attempt to change this ratio by chemical 

 substances gave results which, according to the author, are 

 plainly seen; but he exhibits these effects by drawings rather than 

 measurements, and I find it difficult to convince myseff that there 

 is any difference in the K/P ratio. Hartman also tests the ratio 

 of volume of the nucleolus to that of the nucleus (N/K), and 

 finds that on the whole it changes in the same manner as does 

 K/P. The whole question of the significance of the relative 

 nuclear volume is discussed in considerable detail, in terms of 

 nuclear volume and surface and cell volume and surface, with 

 many formulas, not so much in relation to discovered facts as in 

 relation to facts that may conceivably sometime be discovered. 



Hertwig does not apparently express any opinion of the exten- 

 sion of the idea that the relative nuclear volume is of physio- 

 logical significance to somatic tissues, but he does expressly 

 sanction its application to parthenogenetic species. Nor was the 

 idea a mere passing fancy with Hertwig, for it appears in a series 

 of papers over a period of years. Furthermore, it is clear that he 

 regarded the relative volume of the nucleus as a causal factor, not 

 a mere accompaniment. 



