506 H. S. Jennings and George T. Hargitt. 



turn" forms. From these facts but one conclusion can be drawn, 

 viz., that the two groups not only show a relatively great constancy 

 in the number of micro-nuclei, but the micro-nuclei always differ 

 in structure whether their number is aberrant or normal. Hence 

 the '' caudatum" forms are demonstrably distinct from the '' aurelia" 

 forms even when they have the same number of micro-nuclei. 

 On this basis the probable explanation of Calkins' case is this: 

 An aberrant ''caudatum" form with two micro-nuclei was the 

 starting point of his line; at a later period the individuals in this 

 line returned to the normal condition. That is, Calkins was 

 dealing with P. caudatum only. If this interpretation is correct 

 his conclusion should have been: P. caudatum (with two micro- 

 nuclei) had become P. caudatum again (with one micro-nucleus). 

 This interpretation is borne out by several facts : 



1. It is somewhat remarkable that Calkins in his discussion 

 of this matter, a discussion that includes a list of the differences 

 that Maupas has set forth between the two species ('06, p. 2), 

 does not so much as mention the difference in the structure of the 

 micro-nuclei, which had been emphasized by Maupas and con- 

 firmed by R. Hertwig; 



2. Thus Calkins does not assert that an animal with micro- 

 nuclei having the caudatum structure had changed into one with 

 micro-nuclei having the aurelia structure. 



3. All the figures of micro-nuclei published by Calkins in his 

 recent treatise ('09) and by Calkins and Cull ('07) show the typical 

 caudatum structure. 



4. The size given by Calkins for the animals in which he noticed 

 the change of number of micro-nuclei ('06, p. 6) is throughout 

 that which in our experience is characteristic of caudatum (aver- 

 age size from 178 to 224 microns). No aurelia race studied in this 

 laboratory has ever approached in average size even the smallest 

 of these figures. It should be specially noted that Calkins does 

 not claim that in changing from the number of micro-nuclei 

 characteristic of aurelia (two) to that characteristic of caudatum 

 (one) there was an accompanying change to a larger size, as would 

 be expected if this were an actual change from one type to the 



