208 CLOSE OF THE PRE-EVOLUTIONARY PERIOD 



fore primordial, and we can say with M. Milne-Edwards that 

 everything goes to prove that the distinction established by 

 Nature between animals belonging to different phyla is a prim- ' 

 ordial distinction " (p. 58). 



In other directions also von Baer's work was confirmed 

 and extended by later observers those parts of it particu- 

 larly that had reference to the germ-layer theory, and to the 

 concept of histological differentiation. His germ-layer theory 

 was accepted in its main lines by Rathke, Bischoff and 

 Lereboullet, and applied by them to the multitude of new 

 facts they discovered. Rathke, in particular, was a firm up- 

 holder of the doctrine, and made considerable use of it in 

 his writings. 1 Even before the publication of von Baer's 

 book he had interpreted in terms of the germ-layer theory 

 sketched by his friend Pander the splitting of the blastoderm 

 which occurs in the early development of Astacus, whereby 

 there are formed a serous and a mucous layer, one inside 

 the other like the coats of an onion, to use his own ex- 

 pressive phrase. 2 



An ingenious application of the Pander-Baer theory was 

 made by Huxley, who compared the outer and inner cell- 

 layers which form the groundwork of the Coelentera with 

 the serous and mucous layers of the vertebrate germ. 3 He 

 laid stress, it is true, rather on the physiological than on 

 the morphological resemblance. " A complete identity of 

 structure," he writes, " connects the ' foundation membranes ' 

 of the Medusa; with the corresponding organs in the rest of 

 the series ; and it is curious to remark, that throughout, the 

 outer and inner membranes appear to bear the same physio- 

 logical relation to one another as do the serous and mucous 

 layers of the germ ; the outer becoming developed into the 

 muscular system, and giving rise to the organs of offence and 

 defence ; the inner, on the other hand, appearing to be more 

 closely subservient to the purposes of nutrition and genera- 

 tion " (p. 24). Von Baer had already hinted at this homology 



1 Particularly in his lUcnnius (1833) a "d Natter ( \ 839). 



' In the " preliminary notice "of his Crayfish paper /si's, pp 1093- 

 1 100, 1825. 



' "On the Anatomy and the Affinities of the Family of the Medusre," 

 /'////. Trans., 1849 ; Set. Memoirs, i., pp. 9-32. 



