to its beginning, to trace development retrospectively, and 

 then it is possible to establish the succession of events 

 in actual chronological order. 



The beginning phase of primary differentiation Baer 

 described in the following words: "Initially there are no 

 separable differentiated layers, but only the surfaces of 

 the rudiment, in which one can see differences, as in polyps, 

 indicating the opposition between external and internal 

 surfaces. The space between these surfaces, as in the 

 polyps, is occupied by an indifferent mass" (II, 6(4)i, 

 p. 91 (67-68) ) c The given quotation indicates with complete 

 clarity that the idea of comparing the embryonic layers in 

 vertebrate embryos with the body layers of the coelenterates 

 belongs to Baer, and not to Thomas Huxley, 4 as is frequently 

 claimed. 5 Baer was ahead of the English zoologist by not 

 less than twenty years, if we take into consideration that 

 the printing of his second volume began in 1829. 



In the embryo, Baer continued, the opposition of external 

 and internal surfaces results in differentiation of the upper 

 skin and lower mucous layers. Simultaneously, a differentia- 

 tion occurs along the surface: the middle of the rudiment 

 becomes thin, giving rise to the transparent zone (Wolff's 

 area pellucida) , and the periphery forms the dark zone 

 (Pander's area opaca) . The vascular layer reaches only the 

 middle of the dark zone. Because the vessels are present only 

 in the vascular layer, the periphery is broken up into the 

 internal, vascular zone (Wolff's area vasculosa) which is 

 separated from the yolk zone (area vitellaria) by the broad 

 terminal vein (vena terminalis, which Baer called the sinus 

 terminal) . The transparent zone is also divided: the middle 



Thomas Huxley, "On the anatomy and the affinities of the 

 family of the Medusae," PHIL. TRANS. ROY. SOC, 2 

 (1849) , pp. 413-434. 



See, as an example: I. I. (Ilia or Elias) Mechnikov, 

 EMRYOLOGISCHE STUDIEN AN MEDUSEN. EIN BEITRAG 

 ZUR GENEALOGIE DER PRIMITIV-ORGANE (Wien: 

 A. HcSlder, 1886? Moscow: Akademii Nauk, 1950), 

 pp. 284, 418. 



383 



