W. K. BEOOKS ON THE GENUS SALPA. 57 



an external or peripheral boundary and a growing edge, and that the 

 inclusion of the yolk is a very slow process. 



From Salensky's statement (17), p. 455, that "outside the limits of 

 the germinal area the yolk is perfectly naked," I infer that in Pyrosoma 

 it never becomes completely inclosed by the ectoderm of the cyathozooid, 

 but that this degenerates before the yolk is covered. This secondary 

 adaptation to the presence of a food-yolk is so well understood that we 

 need not dwell on it. So far as I am aware, the salpa embryo is the only 

 one without a food-yolk which is known to have an incomplete ectoderm 

 with a growing edge, and the incompleteness of the floor of the digestive 

 tract is another fact of exactly the same kind. There is therefore good 

 reason for believing that salpa is descended from a form with a food- 

 yolk, like that of Pyrosoma, which has afterwards been lost. The acqui- 

 sition of new methods of nourishing the embryo by means of migratory 

 follicle cells and a placenta is a satisfactory explanation of the way in 

 which the need for a food-yolk was done away with, and its disappear- 

 ance is therefore quite intelligible. 



In considering the influence of a food-yolk upon the structure of the 

 embryo, we must have a clear idea of what the books call its "morpho- 

 logical position." To my mind, the way this term is used by writers on 

 the embryology of vertebrates is open to criticism. The fact that the 

 unconsumed remnant of the yolk of vertebrates is surrounded by endo- 

 derm or inclosed in endoderm cells, is no evidence that it was laid down 

 in the egg in any relation to the regions of the body of the future 

 embryo, or in any "morphological position" whatever. The series of 

 stages in the phylogenetic history of the acquisition of a food-yolk is a 

 series of eggs, not embryos. In secreting the yolk the egg functions as a 

 cell, not as a potential embryo, and there is no reason to believe that the 

 yolk material is laid down in any definite relation to the structure of the 

 latent embryo. Its distribution is probably determined by the structure 

 of the egg as a cell, and the place which the unassimilated remnant 

 occupies in the body of the embryo depends upon the physiological 

 activity of cells of the embryo itself. In fact, as all the cells of a young 

 amphibian embryo are packed with yolk, the yolk of an amphibian egg 

 has no " morphological position." The yolk of Pyrosoma is not inherited 

 from a common source with that of true vertebrates, and there is, of 

 course, no reason why it should be assimilated in exactly the same way. 

 I believe that a careful study of Salensky's figures will show that it is in 

 the body cavity, and not in the digestive cavity of the embryo, for no layer 

 of endoderm ever extends over any considerable part of it. 



