86 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



istic arrangement by means of gravity. From Salensky's ('92, p. 113) 

 statement that in an early stage the changing test cells are separated 

 from one another by a homogeneous substance that looks very much 

 like test matrix, I have thought that he might possibly have had some 

 more refined form of a similar coagulum before him ; but I cannot for a 

 moment suppose that the true nature of a state of affairs such as 

 exists in my preparations would have escaped him. 



The flattening out of the test cells against the ectoderm and test 

 mentioned by Salensky, I find to be due to pressure from without. 

 Wherever the inner follicular epithelium is close to the embryo, as on 

 its sides, for instance, there the test cells are much flattened against the 

 surface (Plate 5, Figs. 32, 34, cl. tst.) ; but where the epithelium is 

 lifted from the surface, as in the triangular space it leaves when passing 

 over the tail, the great majority of the test cells are spherical. Occa- 

 sionally, however, oval ones are found here with their sides next to the 

 ectoderm or test, but in such cases the extreme flattening that occurs 

 on the sides has never been seen. Moreover, in reviewing quite a 

 number of series I have been unable to discover any transitions between 

 the test cells and cells within the test, or any cases in which the periph- 

 eral layer of the test matrix was not perfectly distinct from the 

 superimposed test cells. Thus no evidence at all has been found that 

 is favorable to Salensky's view of the test formation. 



So much for the interpretation of Salensky's results. Concerning the 

 actual method of test development I must say that I think it can be 

 shown that the test in Distaplia is formed in such a way that the test 

 cells cannot be instrumental in producing it. 



The first tunicin, or animal cellulose, that is laid down is that forming 

 the tail fin, which is quite well developed at a time when no other 

 tunicin can be detected either on the tail or the body of the embryo. 

 Now the test matrix of the fin has no cells within its substance, and so 

 must have been formed by the activity of cells that are outside of it. 

 As it is attached to the ectoderm on one of its three sides, and as the 

 test cells are in contact with only about half of the surface of the other 

 two, and are usually not pressed against it, and as these cells are in no 

 way different, from any of the other test cells, there is very little ground 

 for believing the test matrix of the tail to have been secreted by the 

 test cells. 



On the body of the embryo an examination of the early stages shows 

 that here the whole thickness of the test is quite small, much less than 

 the thickness of a test cell, even though the latter is pressed against the 



