Baer considered the irregular growth of fundamental 

 organs as the basic differentiating factor (§ 3 (173)), 

 and he illustrated this idea in particular with examples 

 of development of the skull, cephalic brain and heart. 

 The source of irregular growth Baer saw as in the direction 

 of the flow of nutritional materials, on which depend the 

 separation of: surface layers of the blastoderm from the 

 layers adjacent to the yolk, the central parts from the 

 peripheral parts, and the cephalic part of the embryo from 

 the caudal or tail part. In judgments of this kind, Baer 

 showed his inherent scientific carefulness and stipulated 

 that he did not so much search for the reasons of development 

 as he paid attention to coordinating the processes of 

 development, "that it is for us in the beginning more 

 important to gain knowledge of the deepest basis of the 

 process, because the latter at the beginning of investiga- 

 tions is difficult to clarify in all completeness." This 

 carefulness, moreover, gives credit to Baer that he, in 

 all discussions of the dependence of development on move- 

 ment of nutritional materials, moved beyond the wrong 

 impression that the nutrition of the embryo passed through 

 a canal which connects the central yolk cavity with the 

 blastoderm lying on it. 



Besides the separation of the basic or fundamental 

 organs in the process of development, formations appear 

 which connect the main organs with each other. According 

 to this plan, the sensory organs develop (they connect the 

 nervous system with the skin) and the derivative mucodermal 

 tubes pass outwards (mouth, anus and branchiate slits). 



About histological differentiation, Baer said very 

 little, considering only the differentiation of bones and 

 nerves from muscles. He reasonably objected to Serres, who 

 suggested that nerves grow from the periphery to the center. 

 Yet he also adopted the inaccurate view that nerves are 

 formed at once along their length "from the muscular layer 

 by means of histological separation." Baer did not promote 

 this idea, remarking that the essence of the histological 

 separation remained unknown to him. 



The fifth scholium, entitled "On the relation of forms 

 which the individual takes at the different stages of its 

 development," is most widely known, and most frequently cited. 



348 



