32 THEORIES SINCE 



besides the parent-cells of the coelomic epithelium, the skeleto- 

 trophic mesenchyme (mother -cells of connective tissue, blood, 

 and blood-vessels), traceable probably to endodermal parentage, 

 myoblastic mesenchyme probably derived from both primary layers 

 and neuroblasts derived almost certainly from both primary layers. 

 The parent-cells of the epiblastic nerve centres usually separate 

 together as a distinct mass at a later period of development from 

 the primary ectoderm, but there is abundant embryological proof 

 that so-called " mesoblast " may contain parent-cells of nerve tissue 

 as one of its constituents (e.g. in Cephalopoda). In some cases 

 too the single mother-cells of the nephridia take up their place in 

 the mesenchyme, migrating probably from ectoderm. There is still 

 a very large and very difficult field of research open to the student 

 of cellular embryology. The cell-lineage of mesenchyme and other 

 factors of mesoblast must be determined ; it is not enough to 

 have disentangled coelom from this confused mass. When the 

 cell -lineage of mesenchyme and its tissue products have been 

 cleared up, we shall be able finally to put aside the hasty criticisms 

 and phantastic assertions of those who have grown impatient over 

 the slow and difficult task of Cellular Embryology. 



(d) Third Stage of the Theory of the Coelom from 1896 to the 

 present day. 



A third stage in the progressive adjustment of the theory of 

 the coelom is now in progress ; it has reference to the relation of 

 the ccelom to renal excretory organs. 



It had become abundantly clear in the early days of speculation 

 concerning the coelom that the reproductive cells both male and 

 female are in all Ccelomocoela epithelial cells of the coelomic 

 space. In the attempt to define the coelom this fact was made 

 use of, but it was also r maintained by myself and others that the 

 communication of the coelom with the exterior by at least one 

 pair of renal excretory tubes was characteristic ; and the attempt 

 was made (and not unsuccessfully) to identify a given space as one 

 of coelomic origin by the fact that it was placed in communication 

 with the exterior by means, of sttch renal fexcretory tubes or sacs. 



Led by the principle that it is conducive to an ultimate dis- 

 covery of the truth to assume uniformity of origin for similar 

 structures in diverse groups as a first hypothesis, rather than to 

 assume a multiplicity of origins, I proposed (in 1877, Quart. Journ. 

 Micr. Sci. vol. xvii. p. 429) the name " nephridium" for the simple 

 renal excretory organ, and I took the so-called " segmental organs " 

 of the Earthworm as the type. I identified with this typical 

 nephridium the excretory tubules of Platyhelmia and Eotifera, 

 the renal sacs of Mollusca, the peritoneal funnels and connected 



