Turning to the first step of development (Sch. IV, § 1), 

 Baer remarked that the breakdown in the rudiment and embryo 

 follows one plan, but in different directions. Thus, in the 

 rudiment and the rudiment membrane, differentiation of thick- 

 ness develops along the surface and the length. Finally, the 

 fetal area is most developed in the anterior region, and the 

 vascular in the posterior region. 



The differentiation in thickness leads to the separation 

 of the animal part of the embryo, and also the vascular and 

 mucous layers. Differentiation along the surface delimits 

 the embryonic body and the vascular and yolk fields from each 

 other; in length such important parts as the brain and skull, 

 heart and intestines becomes separated. The transformation 

 of the layers of the rudiment into the embryonic body, 

 according to Baer, proceeds from the surface to the interior. 

 At first, parts of the serous layer are differentiated, then 

 formation of the heart and aorta begins in the vascular 

 layer, and only later the mucous sheet begins to form the 

 digestive tube. Along the length and breadth of the embryo, 

 differentiation proceeds in defined succession from the 

 anterior backwards and from the middle to the periphery. The 

 successive processes of separation of the embryo occur in 

 the same order. The conversion of the layers into tubes 

 is accomplished in vertebrates by a double symmetric develop- 

 ment from one axis (I Sch. IV 2a, 143). 



Baer first stated the idea basic to comparative embryo- 

 logical investigation, that the three methods of differentiation 

 just mentioned are apparently inherent to all animals except 

 the simplest. Also, for the vertebrates, in his opinion, it 

 is characteristic that the processes of formation follow the 

 principle of double symmetry in relation to the longitudinal 

 axis of the body. The upper or animal layer forms a tube by 

 rolling up this layer above the axis of the body. Describing 

 this idea in the chick embryo, he also confirmed it in the 

 frog embryo and expressed his certainty that in the other 

 amphibia, fish and mammals development follows that same scheme. 



After formation of the spinal tube under the cord, which 

 lies along the axis of the body, the abdominal tube forms by 

 the union of two symmetrical halves. In transverse section, 

 the vertebrate embryo assumes the form of a figure eight. 



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