the primary tube-shaped organs it is possible to distinguish, 

 first, a central line and, second, a fusion line by which 

 accretion takes place. The fusion lines of each primary 

 organ correspond to the peripheral borders of these plates, 

 from which the organ is formed. The central line of each 

 primary organ had occupied the center while in the stage 

 of the plates. All the central lines lie in the middle plane, 

 one above the other and close to each other. Only the skin, 

 which has two fusing lines, has no central line. At an early 

 stage of development, all the central lines of the future 

 primary organs are concentrated in the primary zone. 



Turning to the development of the extremities, Baer 

 suggested that both pairs of vertebrate extremities are 

 connected with broad muscular belts and that the basic 

 segments of the extremities and the adjoining muscles form 

 the external muscular layer, including both muscular tubes 

 of the trunk. The formation of the first fold of the 

 extremity, in Baer's opinion, confirmed that suggestion, 

 because it appears from each side as a long common fold 

 which constitutes part of the external muscular tube; this 

 in turn represents a primary organ. 



The described relations Baer illustrated with a schematic 

 drawing (Figure 28) . If we lay flat the plane of the layer 

 of the spinal plates ab" and that of the abdominal plates ac", 

 they will look like ab and ac. When the plates of the 

 extremities b"c M are located in the plane ab be, then the 

 transverse section shows that they extend from the closing 

 line of the back (b" or b) to the closing line of the 

 abdomen (c" or c) . From the scheme it appears that in the 

 zone of the rudiment, through the rise of the spinal plates, 

 the plates of the extremities will be the continuation of 

 the back plates. (II 6A, 77-78) 



385 



