^54 WILSON. [Vol. VI. 



" That there is a general homology appears, on the other 

 hand, hardly open to doubt " (p. 285). 



" The fact of the triploblastic condition being later than the 

 diploblastic proves in a conclusive way that the mesoblast is a 

 derivative of one or both the primary layers. . . . 



"... The mesoblast did not at first originate as a mass of in- 

 dependent cells between the two primary layers, but ... in the 

 first instance it gradually arose as differentiations of the two 

 layers, and ... its condition in the embryo as an independent 

 layer of undifferentiated cells is a secondary condition, brought 

 about by the general tendency towards a simplification of de- 

 velopment, and a retardation of histological differentiation " 

 (p. 286). 



Certain facts, clearly specified by Balfour, point, in his opin- 

 ion, to the establishment of the two following propositions : — 



" (i) That with the differentiation of the mesoblast as a dis- 

 tinct layer by the process already explained, the two primary 

 layers lost for the most part the capacity they primitively pos- 

 sessed of giving rise to muscular and connective-tissue differ- 

 entiations, to the epithelium of the excretory organs, and to 

 generative cells. (2) That the mesoblast throughout the triplo- 

 blastic Metozoa, in so far as these forms have sprung from a 

 common triploblastic ancestor, is an homologous structure " 

 (p. 287). 



The point in this lucid and forcible statement to which I wish 

 to call attention is that Balfoitr did not regard the primary germ- 

 layers as being strictly homologous tvith one another throughout the 

 gastrulas of the triploblastica, or with the layers of the coslenterate 

 body. The reasons given for this conclusion are so convincing 

 as to amount almost to a demonstration. 



We turn now to the views of Kleinenberg as developed five 

 years later in the celebrated work on Lopadorhynchus. In cer- 

 tain respects, as Kleinenberg himself points out, his views coin- 

 cide with those of Balfour. But when carefully examined as a 

 whole they are found to lead logically to a conception that is 

 diametrically opposed to that of Balfour, as may clearly be 

 shown by a juxtaposition of the two views. It should be borne 

 in mind, however, that it is difficult to give an adequate idea 

 of Kleinenberg's views by the citation of a few passages, since 

 they are set forth in a style which, though always entertaining 



