302 



NERVE-TISSUE. 



DEVELOPMENT OF NERVOUS TISSUE. 



Very little is known in regard to the development of the nerve- 

 centers and the nerves. All observers adhere to the idea that the 

 nerve-centers are products of the outer or horny embryonal layer, 

 the epiblast. According to general biological views, this cannot 

 be correct. The nerves, being so closely allied to connective 

 tissue, are offspring developments from the middle embryonal 

 layer, the mesoblast. L. linger * maintains that the medullated 

 nerve-fibers of the brain arise from radiating tracts, in which the 

 cells are arranged in columns ; these columns first have a retic- 

 ular appearance and an investing membrane. He claims that 

 the reticulum of the myeline layer is produced earlier than the 

 axis-cylinder ; that connective tissue and nerve- tissue may arise 

 from one and the same cell, and that nerve-fibers or axis-cylinders 

 may come from certain portions of the reticulum. 



FIG. 129. BRAIN OF A HUMAN EMBRYO, FIVE WEEKS OLD. 



P, gray substance with numerous nuclei ; A, axis-cylinders ; C, capillary blood-vessels. 

 Magnified 600 diameters. 



My own observations prove that the brain of a human embryo 

 five weeks old is composed of a bioplasson reticulum, whose 

 points of intersection with low powers appear to consist of gran- 

 ules. In this reticular mass numerous nuclei are imbedded, and 

 tracts of axis-cylinders laid before even a trace of a medullated 

 nerve can be demonstrated. (See Fig. 129.) 



This shows that the development of the nerves can never be 

 understood in accordance with the cell theory, inasmuch as there 



* " Untersuchungen iiber die Entwicklung der eentralen Nervengewebe." 

 Sitzungsber. d. Wiener Akad. d. Wissensch, 1879. 



