318 Origin and Migration of the Germ-Cells in Acanthias 



It is interesting to note that just before the Ureier are taken into 

 the mesoderm tHey all lie near the junction of the three germ layers. 

 It may be that rejDroductive cells which have as they do, possibilities 

 for formation of all three layers can come only from a region in the 

 embryo in which the somatic cells themselves have possibilities in one 

 of these three directions. 



With regard to its bearing on the theory of the gonotome, the evi- 

 dence is, of course, opposed to such a theory. 



As we trace to its early condition, the sexual gland, we do not find it 

 originally segmented. On the other hand, it is first unsegmented and 

 subsequently becomes segmented, because it must, since it moves into 

 the body cavity which is itself segmented. 



SUMMAEY. 



To summarize: The most important conclusion appears to be that 

 the germ-cells in the dog-fish are not developed from somewhat special- 

 ized cells of the body, but that a few undifferentiated cells of the earliest 

 type are taken out and passed on until the new^ individual is formed. 



This early appearance of germ-cells in vertebrates is not a new dis- 

 covery, since it has been observed by Eigenmann in Cymatogasta, 91, 

 a bony fish. Eigenmann, however, considers this fact to be in some 

 way associated with peculiarities of other sorts in this fish. 



J. Beard, 00, has announced the early appearance of the germ-cells 

 in Eaja batis, the skate, though his paper containing the proofs is still 

 wanting. 



In the lamprey, according to W. M. Wheeler, 99, the germ-cells are 

 not derived from the mesothelium cells, but appear very young in the 

 endoderm. 



, In all these forms cited above, the germ-cells had not been studied 

 at all prior to these investigations, so it was not so much that the epi- 

 thelial origin was overthrown as that such origin was not the true one 

 in these special cases. 



The value of finding a similar condition in the dog-fish and shark 

 seems to me to be twofold — first, because these have long been used 

 so much by investigators and students as typical lower vertebrates, and 

 second, it shows that even among them where an epithelial origin was 

 contended the early endoderm origin is likewise true, thus bringing 

 them all in the same category. 



Lastly, its bearing on the continuity of the germ plasm. Moritz 

 ISTaussbaum, in 1880, formulated an hypothesis that "the sexual cells 



