itself is composed of two layers, and the embryonic 

 membrane separates the internal layer from the external; 

 this suggestion is contrary to the fact of the disappearance 

 of the yolk membrane above the vascular area. 3) The 

 blastoderm is formed from the submerged yolk membrane. This 

 possibility Oken considered the most probable, but it is an 

 entirely arbitrary suggestion, not confirmed by direct 

 observation. 



Oken continued to remark that he did not understand 

 the formation of the two-layered blastoderm, and asked what 

 is present between it and the yolk membrane. 



Referring to Pander's description of the formation of 

 the primary folds, Oken considered the most important part 

 of the dissertation to be that on the ptcnatwn saliens.^l 

 He expressed regret only that these parts of the embryo are 

 not illustrated in the schematic ("ideal") drawings. (73) 

 He stated in this respect that "Each prepared anatomical 

 drawings should follow the rule of depicting things not as 

 they look, but as they exist. The so-called drawing from 

 nature is always an expression of what seems to be. The 

 true vision is not that of the artist, but that of the 

 philosopher" (p. 1533) . Concerning the drawings accompanying 

 Pander's German text, Oken said that they allow no under- 

 standing of the work because they are not schematic. 



A number of details remained unclear to Oken: for 

 example, the topography of the layers or folds in relation 

 to the blastoderm, and the yolk to the shell. To him the 

 method of formation of the spinal cord is not clear; is it 

 a tube or a groove? Oken's perplexity is completely logical 

 for Pander described the gradual closing of the spinal folds, 

 between which from the beginning there is already a thread- 

 like spinal cord. The solution to this confusion was 

 achieved only by Baer, who showed that the thread is not the 

 spinal cord, but a cord where the brain is formed as a 



31. Punctum sali-ens — a springing point. This is what 



Aristotle named the rudiment of the heart of the chick 

 embryo, from which, in his opinion, the processes of 

 formation begin. See Aristotle, THE DEVELOPMENT 

 OF ANIMALS, Second Book, 4 (Academy of Science, 

 USSR, 1940) , p. 109. 



261 



