108 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



or nine, a part of which was contained in each of two sections. If these 

 undifferentiated mesoderm cells had been developed from kalymmocytes, 

 we should not expect to find them thus massed together, but, instead, 

 distributed at intervals approximately corresponding to the distribution 

 of the kalymmocytes about the embryo. Nor should we expect to find 

 them penetrating other kalymmocytes. It must be admitted, however, 

 that neither of these facts is wholly inconsistent with Salensky's theory. 



3. Undifferentiated mesoderm cells were found on the surface of 

 younger embryos, such as had not yet secreted any test-matrix, as well 

 as on the surface of older ones possessing a test of considerable thick- 

 ness. But while in these older embryos there were transitions to cells 

 that were highly vacuolated, like the kalymmocytes, in the younger ones 

 these transitions were entirely absent. This is exactly what should be 

 expected according to my interpretation, for the cells on the younger 

 embryos have just wandered out through the ectoderm, and have not 

 yet had time to become vacuolated, as they have in the older embryos. 

 I do not see how, according to Salensky's view, these facts can be ex- 

 plained, for if the undifferentiated mesoderm cells have been derived from 

 the kalymmocytes, the transition stages should be present in both cases. 



4. The strongest objection to my theory, and one that will immedi- 

 ately present itself to every one, is the great improbability that cells 

 which have had such a different history as typical mesoderm cells and 

 kalymmocytes should both end in structures that cannot be distin- 

 guished from each other. It must be remembei'ed, however, that both 

 are of mesodermic origin, and that both are subjected to the same envi- 

 ronment. Improbable as such a convergence may seem, it does, how- 

 ever, occur in D. magnilarva ; for, in Professor Salensky's sections, I 

 have seen in the body cavity cells, certainly not displaced by the knife 

 in sectioning, which were vacuolated in such a way that only with diffi- 

 culty could they be distinguished from kalymmocytes, — one, indeed, 

 that had exactly the appearance of a kalymmocyte. The a priori im- 

 probability of such a convergence in development is thus shown to be 

 without weight. On the whole, then, I think it must be said that, in 

 spite of the series of transitions from kalymmocytes to cells resembling 

 those in the test (D. magnilarva), this series does not prove that the 

 kalymmocytes participate in the formation of the test. 



Berlin, March, 1899. 



