but only in another direction. Is the nucleus causing the 

 division? Baer confirmed that "this I could not establish 

 conclusively in the frog, because its nuclei are excessively 

 dark and large. Later this process was clarified for me in 

 the ova of the sea-urchin, which I investigated in Trieste 

 during a journey from Petersburg" (see Chapter 23) . 



The phenomena of division of the frog ovum gave Baer 

 the grounds to speak decisively about preformation. "The 

 history of metamorphosis of the yolk sphere in amphibia," 

 Baer wrote, "had clearly solves this important question, 

 which was for me an unexpected joy. "15 He did not consider 

 it appropriate in the special works of his volume to discuss 

 the old moot point about the preexistence, or epigenesis of 

 a new individual . Baer noted that although studies about 

 preformation, assuming the preexistence of a new organism 

 up to fertilization, had long been considered an unfounded 

 fantasy, all questions cannot be solved conclusively by 

 direct observation. In truth, the new embryological inves- 

 tigations supported the status that all separated parts of 

 the new individual are formed as a result of transformation 

 of earlier formed more general parts. Hence, the animal 

 section of the vertebrate body is formed from the outer or 

 animal layer of the rudiment, and the vegetative parts are 

 formed from the lower (internal) layer. Thus the embryo is 

 considered the result of isolation of a part of the rudiment 

 It seems that the rudiment is, without question, the undevel- 

 oped animal. However, the formation of the rudiment up to 

 fertilization precedes the formation of the less determined 

 organized mass, which clearly differs from the proper yolk. 

 Baer called this mass the rudimentary layer; in the non- 

 fertilized frog ovum, the dark cover in one of the sides of 

 the ovum is considered this rudimentary layer. The rudimen- 

 tary layer is used for the formation of rudiment as a single 

 unity, and not as a substance. The rudimentary layer in the 

 frog ovum up to fertilization is continuous, but from the 

 beginning of development its continuity is disturbed with 

 each division of the ovum. 



15. Baer, "Die Metamorphose des Eies der Batrachier, " p. 506 



427 



