BANCROFT : OVOGENESIS IN DISTAPLIA OCCIDENTALS. 107 



tinguished from a kalymmocyte. The facts for and against this interpre- 

 tation may be briefly stated as follows : — 



1. The fact that, among the cells entirely outside the embryo, the 

 series may be traced not only to the typical vacuolated cell of the test, 

 but also to the undifferentiated mesoderm cell, tells against Salensky's 

 view that the kalymmocytes become rejuvenated and help to form the 

 test. For, if his view were correct, one would expect that the rejuve- 

 nation of the kalymmocytes would cease as soon as they had reached 

 the structure of the typical cell of the test, and that the concentration 

 of the cytoplasm would not go farther, and pi-oduce cells not to be dis- 

 tinguished from undifferentiated mesoderm cells. Undifferentiated meso- 

 derm cells are found in the test in comparatively small numbers ; and, 

 as it is certain that at least some of them wander through the ectoderm 

 in the undifferentiated condition, and later undergo vacuolation to form 

 the typical cells of the test, it seems probable that sooner or later all 

 the undiffei'entiated mesoderm cells within the test do the same. If 

 this is true, then the undifferentiated mesoderm cells found on the out- 

 side of the test are, according to Salensky's theory, destined to reverse 

 their development once more and become again moderately vacuolated, 

 after having already passed through that stage twice. Of course the 

 considerations adduced in this paragraph do not prove my point, for it 

 may be replied that, as the participation of the kalymmocytes in the 

 building up of the test implies a rejuvenation, it is not at all surprising 

 that some of the cells should become more rejuvenated than is neces- 

 sary. The above considerations, however, show that the presence of 

 undifferentiated cells on the outside of the test adds a further compli- 

 cation to Salensky's theory. 



2. These undifferentiated mesoderm cells are found in such positions, 

 and associated with each other in such a way, that it is difficult to see 

 how they can have been formed from kalymmocytes. Thus, in one case, 

 in an embryo in which none of the cellulose test-matrix had been secreted, 

 an undifferentiated mesoderm cell was found within the cytoplasm of a 

 typical kalymmocyte, whose shrivelled nucleus was still distinctly visi- 

 ble. Near by, but still imbedded in the ectoderm, was another mesoderm 

 cell of exactly the same appearance, as if to show the course which the 

 first cell had traversed. 



In several other places on the surface of the same embryo isolated 

 undifferentiated mesoderm cells were found. The majority, however, of 

 the mesoderm cells thus situated were aggregated into groups. In one 

 case a group of three was encountered, and in another a group of eight 



