214 Proceedings of the Boyal Physical Society. 



look whether or not data afforded by it are such as to affect 

 in any way our notions of the general constitution of the 

 vertebrate body. 



In recent years, owing in great part to the efforts of 

 Sedgwick, our ideas on the " cell theory " have become 

 somewhat modified. Sedgwick emphasised the fact that the 

 study of vertebrate embryos shows that the cells forming 

 the body are, as a rule, not isolated, but joined together by 

 bridges of protoplasm. The study of such a form as Lepido- 

 siren, with its enormous cell elements, brings out very clearly 

 the truth of Sedgwick's statements as regards, e.g., the 

 mesenchymatous network. We still have a cell theory. We 

 still look upon a vertebrate as consisting of vast numbers of 

 cells. It is not, however, to be looked on as an aggregation 

 of cells in the literal sense, but rather as a mass of living 

 matter subdivided into cells. It does not grow because its 

 cells multiply, but rather its cells become more numerous 

 because it increases in size. The division of a developing 

 metazoon into cells is to be looked on as a necessary conse- 

 quence of the increase in volume of its living matter, very 

 much as Sachs long ago taught was the case with the grow- 

 ing point of one of the Metaphyta. 



I was greatly struck by observing in the ectoderm of young 

 Lepidosirens beautiful tailed cells, recalling exactly the tailed 

 cells familiar to us in the neuroepithelial and myoepithelial 

 cells of the Coelenterata. So far as I can judge, these tail- 

 like processes pass into a kind of plexus lying beneath the 

 ectoderm, and more or less mixed up with the basement 

 membrane. Processes from the underlying mesenchyme cells 

 also pass up into the plexus. I am inclined to the belief that 

 subsequent research will show that the subepidermic plexus 

 is of high importance in the development of nerves such as, 

 e.g., the lateral nerve, which appear to arise by a process of 

 splitting off from the lower surface of the ectoderm. Tailed 

 epithelial cells, such as I have mentioned, are not found in 

 the ectoderm only. They are beautifully seen in Lepidosiren 

 in the developing myotome. Here, as I have shown else- 

 where, the muscle cell exists for a time as a typical tailed 

 epithelial cell, within the protoplasm of whose body the 



