If one turns to the history of theoretical ideas about 

 development, Baer said, then it can be confirmed that some 

 used observations on frog development for the idea of preexis- 

 tence. Specifically, on the ova of the frog Swammerdam had 

 tried to show that the dark cover of the ovum is directly 

 transformed into the tadpole. Swammerdam was so imbued with 

 this certainty that he declared that the formed frog embryo 

 which he discovered had preexisted before fertilization. 

 Referring to his statement about the disturbance of continuity 

 of the rudimentary layer during division, Baer confirmed that 

 the frog ova provides a basis for refutation of the theory of 

 preformation. 



For examination of Baer's data concerning the subsequent 

 development of amphibia, it is necessary to return to the 

 concluding section (Volume II 11) of his UBER ENTWICKLUNGS- 

 GESCHICHTE. Following the scheme of development he had 

 presented for birds and mammals, Baer assumed that in amphibia, 

 division of the rudiment into two vegetative layers takes place. 

 In this primary stripe which forms from the vegetative layer 

 (if we speak according to the language of recent embryology, 

 then we speak about the roof of the primary intestine) , the 

 spinal cord forms, which is so thick that from embryos condensed 

 in nitric acid it can be removed by the fingers. Next Baer 

 described the formation of spinal shafts, which are widely spaced 

 at first, and narrowly spaced later; they ascend as high edges and 

 bend towards each other. At the time of closure of the spinal shafts 

 their internal layer is separated, so that the cerebrospinal canal is 

 formed from two intergrowing layers. In the anterior part of this 

 canal, until its accretion, are seen dilations which represent th* 

 future cerebral cavities. All these processes, Baer noticed, can~be 

 seen more clearly in amphibia than in birds and mammals. 



Baer undoubtedly saw the phenomenon of epiboly of the 

 ectoderm, since he comments on how "the rudiment till the 

 formation of the embryo (that is to say up to neuralization) 

 covers nearly all the yolk sphere." In connection with this, 

 he questioned the comparison with the meroblastic ova, asking 

 "Is the entire rudiment becoming the embryo, or is the rudiment 

 divided into two parts, the embryo and the blastoderm?" 



The answer to this question is: "since the umbilicus 

 is not formed, then gradually all the rudiment becomes the 



428 



