256 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



sections show detached scales of considerable size, but more generally 

 the cells separate from each other and are thrown off singly. Barrois 

 says that he does not know what becomes of the epithelial capsule after 

 it is detached, although he thinks that it may possibly become converted 

 into the cuticle of the embryo. 



Salensky believes that this foetal structure which, in its origin, is not 

 part of the embryo but part of the body of the chain-salpa, becomes con- 

 verted into the ectoderm of the embryo, which is therefore derived, not 

 from the egg, but from the wall of the atrium, or, as he states, that of 

 the pharynx. 



His account is so irreconcilable with my own that I shall quote it at 

 length. 



He says (5), p. 135, that, in Salpa pinnata, "The integument is 

 developed out of the Ectodermkeim, which must therefore be regarded 

 as a structure analogous to the ectoderm of other animals. We have 

 seen that in the preceding stages the Ectodermkeim consisted, in the 

 greater part, of a single layer of small flattened cells which become 

 thickened and cubical at the top of the embryo. At a later stage the 

 thickened area of ectoderm cells grows farther and farther down, until, 

 over the whole surface of the embryo, the ectoderm cells have regained 

 their original, cubical form. The cause of this change in the form of 

 the ectoderm cells is, in my opinion, to be found partly in the difference 

 in the rate of growth of the internal parts of the embryo as compared 

 with the ectodermal layer, and partly in the production of the cellulose 

 mantle. At the time when the ectoderm cells become flattened, we find 

 that the mesoderm cells (cells of the somatic layer of the follicle) are mul- 

 tiplying, and thus greatly increasing the size of this layer. After the pro- 

 liferation of the mesoderm cells has come to an end, and the secondary 

 cavity of the follicle has become filled up with these cells, the growth of 

 the ectoderm begins ; and this stands in obvious relation to the forma- 

 tion of the cellulose mantle." 



"This latter is formed very early in Salpa pinnata, and its first stage 

 is the division of the cells of the Ectodermkeim, which precedes the 

 growth of the ectoderm cells. I need hardly note that the cellulose 

 mantle is formed from ectoderm cells, as Hertwig, Arsenieff, Todarro 

 and the author have shown. In Salpa pinnata we find before the forma- 

 tion of the cellulose layer, the division of the ectoderm cells which sepa- 

 rate later from the ectoderm and pass into the clear homogeneous 

 cellulose. This layer is first formed at the upper end of the embryo 



