ORIGIN OF THE GERMINAL LAYERS. 359 



The Hertwigs have recently suggested in their very interesting memoir 

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

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

 Chastopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinoder- 

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

 the Rotifera, and Platyelminthes. 



The Enteroccela are forms in which the primitive alimentary diverticula 

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

 system has originated from the epithelial walls of these diverticula, part 

 however being in many cases also derived from the amoeboid cells, called by 

 them mesenchyme, by the second process of mesoblastic differentiation men- 

 tioned on p. 347. 



In the Pseudoccela 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 

 derived from mesenchyme cells. That in many cases, and strikingly so in 

 the Chordata, the muscle cells retain clear indications of their primitive 

 origin from epithelial cells, I freely admit ; but I do not believe either that 

 its histological character can ever be conclusive as to the non-epithelial 

 origin of a muscle cell, or that its derivation in the embryo from an indiffer- 

 ent amoeboid cell is any proof that it did not, to start with, originate from an 

 epithelial cell. 



I hold, as is clear from the preceding statements, that such immense 

 secondary modifications have taken place in the development of the meso- 

 blast, that no such definite conclusions can be deduced from its mode 

 of development as the Hertwigs suppose. 



In support of the view that the early character of embryonic cells is no 

 safe index as to their phylogenetic origin, I would point to the few following 

 facts. 



(1) In the Porifera and many of the Ccelenterata (Eucope polystyla, 

 Geryonia, &c.) the hypoblast (endoderm) originates from cells, which accord- 

 ing to the Hertwigs' views ought to be classed as mesenchyme. 



(2) In numerous instances muscles which have, phylogenetically, an 

 undoubted epithelial origin, are ontogenetically derived from cells which 

 ought to be classed as mesenchyme. The muscles of the head in all the 

 higher Vertebrata, in which the head cavities have disappeared, are examples 

 of this kind ; the muscles of many of the Tracheata, notably the Araneina, 

 must also be placed in the same category. 



(3) The Mollusca are considered by the Hertwigs to be typical Pseudo- 

 ccela. A critical examination of the early development of the mesoblast in 

 these forms demonstrates however that with reference to the mesoblast they 



