296 EVOLUTION OF THE ME SO BLAST. 



of these walls. This view, which was originally put forward by 

 myself (No. 260), appears at first sight very improbable, but if the 

 statement of the Hertwigs (No, 270), that there is a large develop- 

 ment of a hypoblastic muscular system in the Actinozoa, is well 

 founded, it cannot be rejected as impossible. Lankester (No. 279), 

 on the other hand, has urged that the mode of origin of the meso- 

 blast in the Echinodermata is more primitive ; and that the amoeboid 

 cells which here give rise to the muscular and connective tissues re- 

 present cells which originally arose from the whole inner surface of 

 the epiblast. It is, however, to be noted that even in the Echino- 

 dermata the amoeboid cells actually arise from the hypoblast, and 

 their mode of origin may, therefore, be used to support the view that 

 the main part of the muscular system of higher types is derived from 

 the primitive hypoblast. 



The great changes which have taken place in the development 

 of the mesoblast would be more intelligible on this view than on the 

 view that the major part of the mesoblast primitively originated 

 from the epiblast. The presence of food-yolk is much more frequent in 

 the hypoblast than in the epiblast ; and it is well known that a large 

 number of the changes in early development are caused by food-yolk. 

 If, therefore, the mesoblast has been derived from the hypoblast, 

 many more changes might be expected to have been introduced into 

 its early development than if it had been derived from the epiblast. 

 At the same time the hypoblastic origin of the mesoblast would 

 assist in exp'aining how it has come about that the development of 

 the nervous system is almost always much less modified than that 

 of the mesoblast, and that the nervous system is not, as might, on 

 the grounds of analogy, have been anticipated, as a rule secondarily 

 developed in the mesoblast. 



The Hertwigs have recently suggested in their very interesting memoir 

 (No. 271) that the Triploblastica are to be divided into two phyla, (1) the 

 Enterocoela, and (2) the Pseudocoela; the former group containing the 

 Chsetopoda, Gephyiea, Brachiopoda, Nematoda, Arthropoda, Echinoder- 

 mata, Enteropneusta and Chordata ; and the latter the Mollusca, Polyzoa, 

 the Rotifera, and Platyelmiiithes. 



The Enterocoela are forms in which the primitive alimentary diver- 

 ticula have given origin to the body cavity, while the major part of the 

 muscular system has originated from the epithelial walls of these diver- 

 ticula, part however being in many cases also derived from the amoeboid 

 cells, called by them mesenchyme, by the second process of mesoblastic 

 differentiation mentioned on p. 287. 



In the Pseudocoela the muscular system has become differentiated from 

 mesenchyme cells ; while the body cavity, where it exists, is merely a 

 split in the mesenchyme. 



It is impossible for me to attempt in this place to state fully, or do 

 justice to, the original and suggestive views contained in this paper. The 

 general conclusion I cannot however accept. The views of the Hertwigs 

 depend to a large extent upon the supposition that it is possible to dis- 

 tinguish histologically muscle cells derived from epithelial cells, from those 



