NOTES ON EMBRYOLOGY AND CLASSIFICATION. 421 



we may have many varieties as to the exact time and mode 

 of such segregation ; and thus, I think, we may explain the 

 various modes of origin which careful observers ascribe to 

 the musculo-skeletal tissues in various cases. To return, for 

 example, to the case of the Vertebrates : it appears that the 

 whole, or very nearly the Avhole, of the musculo-skeletal 

 tissues in those animals are the progeny of the endodermic 

 cells, that is to say, develop from the parenteric outgrowths. 

 Our hypothesis of precocious segregation explains this, for 

 we have only to suppose that, during the first cell-division of 

 the egg-cell, the precociously segregated musculo-skeletal 

 molecules do not form distinct isolated cells, hut accompany 

 the endodermic cells, and do not segregate from these latter 

 until they have formed the parenteric cell-masses. Thus the 

 parenteric cell-masses of Vertebrata, whilst they represent 

 the gastro-vascular diverticula of the Cffilenterate-phase of 

 animal evolution, contain at the same time the hereditary 

 musculo-skeletal molecules. Hence the mesoblast of Verte- 

 brates represents in form the coelomic diverticula, whilst in 

 substance it also is the representative of the musculo-skeletal 

 tissue, primarily differentiated by delamination of an ecto- 

 derm. In this way we are able to explain — that is, to form a 

 plausible conception of the mechanism of — two very puzzling 

 embryological phenomena, hy means of one hypothesis. The 

 phenomena are the formation of an enteron, sometimes by 

 Delamination, sometimes by Invagination with a blastopore, 

 and the formation of the musculo-skeletal tissues, sometimes 

 by delamination from the ectoderm, sometimes by outgrowth 

 of the enteron. 



Among the many variations possible in the origin of meso- 

 blast — musculo-skeletal i\s&\xe plus coelomic epithelium — we 

 may note that the one factor of that double entity, viz, the 

 coelomic epithelium, can always be traced to the enteron or 

 to the primitive enteric cell, whilst the other factor may be 

 (1) wholly or (2) partially fused, as we have above explained, 

 with the enteron, or (3) entirely independent of it. Such 

 part of the musculo-skeletal factor as is not appropriated by 

 the enteron 7nay still continue to arise by delamination from 

 a fully-formed ectodermic cell-layer, or may appear in the 

 very early stages of development as independent cells, having 

 segregated before the cell-division of the embryo had ad- 

 vanced very far. In Pisidium, Paludina, Limneeus, and 

 other Invertebrates, it appears very probable tbat, whilst a 

 large part of the musculo-skeletal tissues arise from the 

 parentera with which their elements have become associated 

 through precocious segregation, other parts of the muscula- 



