ON THE CELLULAR THEORY OF DEVELOPMENT. 91 



The So-called Mesenchyme Tissue of Elasmobranch 



Embryos. 



This tissue is always described as consisting of branched 

 cells lying between the ectoderm and the endoderm. The 

 cells are spoken of as being separate from one another, and 

 from the adjacent ectoderm and endoderm, excepting at points 

 where they are supposed to arise from one of the primary 

 layers. And not only are they described as being separate 

 cells, but they are actually drawn in the author^s figures as 

 separate from each other. This is, perhaps, the best instance 

 that can be given of the bondage in which the cellular theory 

 holds its votaries. For what are the facts? The separate 

 cells have no existence at all ! In their place we find, on 

 looking into the matter, a reticulum of a pale non-staining 

 substance holding nuclei at its nodes. It is these nodes, with 

 their nuclei, which are drawn by authors as the separate 

 branched cells of the mesenchyme, and they are constrained 

 by this theory, with which their minds are saturated, not 

 only to see things which do not exist, but actually to figure 

 them. Another erroneous view due to the same cause is the 

 view that this mesenchyme tissue is not continuous with the 

 ectoderm or with the endoderm ; whereas, as a matter of fact, 

 the opposite is the case, for the primary layers are simply parts 

 of this reticulum in which the meshes are closer and the nuclei 

 more numerous and arranged in layers. These are facts of 

 which anyone with an unbiassed mind can convince himself by 

 the simple inspection of a Selachian or an Avian embryo, and 

 they would have been recognised long ago had it not been for 

 the dominating influence of the cellular theory of development. 



The current views as to the origin of this tissue show just 

 as conspicuously the influence of the same theory. It is said 

 to arise by the budding-off and migration of cells from the 

 walls of the embryonic ccelom, from the primitive streak, and 

 from the neural crest ; and the space between the ectoderm and 

 endoderm into which these cells migrate is described as being 

 empty of structural elements. What are the facts? The 



