LITERARY NOTICES. 



Histogenesis and Combination of Nervous Elements. (•) 



At the time of its separation, the medullary plate consists of a single- 

 layered epithelium. The cells s'^on begin to vary, some enlarging at the 

 peripheral portion and others entad. The nuclei in each case move to- 

 ward the larger end of the cell and thus form two or more rows, though 

 remaining nearer the middle than the ends of the cells. 



The body of the cells soon differentiates into a transparent fluid or 

 gelatinous substance and a more dense granular or striated portion. 

 Usually the first step in this differentiation is the formation of vacuoles 

 which, as they increase in size, tend to coalesce. 



The denser reticular portion accumulates in the peripheral portion of 

 the cells. Thus, briefly stated, the original epithelium cells form a 

 frame-work, which His calls the myelo- or neuro-spongium, the indi- 

 vidual cells being spongioblasts. The nucleated bodies of the spongio- 

 blasts form a broad medimi zone, while the outer and inner margins are 

 devoid of cells. The inner or columnar zone consists of longitudinally 

 striate columns, which expand at the inner surface to form a continuous 

 marginal layer. The outer marginal or mantle zone (randschleier) con- 

 sists of a reticulum of fibres which is penetrated by radiating pillars 

 expanding at the periphery. 



The nervous elements appear at an early stage in spaces between the 

 ventricular part of the epithelium cells in the form of germinal cells of 

 uncertain origin. The number varies with the stage of development, 

 being especially abundant at the time of the appearance of the first 

 nerve roots. The period of maximum development also varies with the 

 locality, being earlier in the cervical than the cerebral or lumbar regions. 

 The number diminishes gradually and this reduction takes place earlier 

 in the ventral than the dorsal half of the medullary plate. 



These germinal cells at a definite time begin to change their form, 

 becoming acute peripherally, until they give rise to a more or less 

 thread-like process, which becomes the nerve fibre, connected at its base 



I Wm. His. Histogenese und Zusammenhang der Nerven-elemente. Referat in 

 der anatomischen Section des internatioiialen niedicinischen Congresses zu ISerlin. Sitz- 

 ung von 7 August, 1890. Archiv. f. Anat. u. Phys., 1890. Supplement-Band. p. 95. 



