As already noted, the central part of the nervous 

 system develops, in Baer's opinion, by primary differ- 

 entiation through exfoliation from the internal surface 

 of the spinal plates, while the peripheral part is formed 

 through histological differentiation of the muscular 

 layer. At the beginning, the tissue differentiating into 

 the neural tube is characterized by histological homo- 

 geneity, but soon differences in the structure of the 

 surface and in deeper layers appear; particularly in the 

 latter which connects with brain tissue. The division of 

 the neural tube into rudiments of the brain and spinal 

 cord takes place before the fusion of the spinal plates, 

 insofar as the anterior part of the tube appears wider 

 than the posterior. 



The spinal cord and its extension maintains a regular 

 thickness, with the exception of the places of formation 

 of the extremities, where there are thickenings of the 

 spinal-brain tube. The internal structure of the spinal 

 cord becomes visible gradually. In it there appear four 

 main main structures, which are particularly obvious at 

 the internal surface. The number of structures increases, 

 and still later fibers become recognizable in it. The 

 internal part becomes grey, and the external white, while 

 the grey matter at the transverse section acquires the 

 form of a cross. 



The cephalic brain at the early stages of development 

 is little different from the spinal cord. Yet, it must not 

 be thought, Baer said, that the cephalic brain represents 

 an anterior extension of the spinal cord into the skull 

 cavity, or the reverse. They both represent a modification 

 of one primary organ, the brain tube, and are formed from 

 it by means of morphological differentiation. The earliest 

 part of the cephalic brain soon divides into separate 

 portions, each of which expands to form brain vesicles; 

 between them interceptors form. At first the anterior 

 vesicle separates from the most elongated posterior one, 

 then the latter subdivides into two and produces the stage 

 of the three vesicles; the anterior, middle and posterior. 

 The anterior vesicle represents the future large brain, the 

 posterior becomes the cerebellum and the medulla oblongata, 

 and the middle becomes the four-hil locked mass. The anterior 



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