602 MANUAL OF HISTOLOGY. 



plexus and pineal gland. It consists of very irregular granules, ranging 

 in diameter from 0'0113 to 0-5638 mm. of flattened or more generally 

 spheroidal, or at times most fantastic shape. These masses, formed of 

 concentrically arranged layers of carbonate of calcium, with phosphate of 

 calcium and magnesium combined with an organic substratum, are usually 

 imbedded in bundles of connective-tissue, in which they present a dark 

 outline. Their occurrence is almost entirely confined to the human brain, 

 and much doubt exists as to their histological significance. 



Turning now to the origin of the central organs in the embryo, it will 

 be remembered that both brain and cord are developed from the so-called 

 corneous germinal plate, that is, from that portion of it bordering on the 

 middle long axis of the embryo, and named, accordingly, by Rema'k, the 

 medullary plate. Properly speaking, it is more the province of embryo- 

 logy than of histology to follow up the transformation of this portion ot 

 the germ into a groove, and the subsequent closure of the latter. This 

 much, however, may be mentioned here, that at an early period the still 

 wide central canal of the spinal cord is lined by a layer of grey substance 

 composed of small cells of round form. These elements gradually become 

 increased in number at the spot where the anterior cornu exists later on, 

 and from this point the nerve fibres of the anterior roots make their exit. 

 The white portions of the cord are developed later on, but their mode of 

 origin and relation to the grey substance requires further investigation. 

 The fibres of the sensitive roots are formed at the same time with the 

 development of the posterior column. Both the lining epithelium and 

 subjacent layer of sustentacular tissue are distinctly visible at an early 

 period : the former is originally thick, and made up of several layers. 



At present we possess but a very fragmentary knowledge of the histo- 

 genesis of the brain and its appendages. The important question, like- 

 wise, as to the origin of the connective-tissue framework of the nervous 

 centres is especially difficult to answer. 



Two species of cells were found by Boll, even at a very early period, 

 in the cortical layer in the embryonic chick, one with vesicular nuclei 

 and sharply defined body, the other with bodies hardly distinguishable 

 from the surrounding protoplasm. 



From the first species the ganglion corpuscles are developed ; from the 

 second the cellular elements of the sustentacular substance. From the 

 first a number of ramifications, with varicose fibres, may be seen to spring ; 

 the latter present, after a time, halo-like rings of the peculiarly consti- 

 tuted neuroglia around them. 



In the early brain of birds the white substance presents bundles of 

 extremely fine fibrillse, separated by long rows of roundish, polygonal, 

 flattened, nucleated cells. From the first the sustentacular matter is 

 developed. The second, formed of spindle cells with two long varicose 

 fibres at the poles, attract around them, later on, a number of the granules 

 of the nervous medulla, and by the fusing together of the latter the 

 medullary sheath is produced. 



The envelopes, blood-vessels, and lymphatics of the brain and cord are 

 developed from the middle germinal layer. The blood-vessels may be 

 beautifully seen, advancing into the substance of the brain and spinal 

 medulla, in the form of bud-like excrescences from the envelopes, and 

 spreading out and uniting in their interior (His). 



