THE SPINAL MARROW. 



749 



developed out of the cells composing the grey matter or from separate 

 blastema to which the mesoblast may in part contribute. It is 

 combined with connective tissue elements, and its structure is <lifferent 

 from that of the grey substance, which is undoubtedly produced by 

 multiplication and differentiation of the involuted epiblastic cells. 



Flj:. 549. 



Fig. 549.— Traksvekse Section op Half 

 THE Cartilaginous Vertebral Column 

 AND THE Spinal Cord in the Cervical 

 Part of a Human Embryo op from 

 NINE to ten weeks (from Kolliker) '~ 



c, central canal lined with epithelium ; 

 a, anterior column ; ^j, posterior column ; 

 p', band of Goll ; [/, ganglion of the 

 posterior root ; p r, posterior root ; a r, 

 anterior root passing over tlie ganglion ; 

 d m, dura-matral sheath, omitted near p r, 

 to show the posterior roots ; b, body of 

 the vertebra ; cl), chorda dorsalis ; n a, 

 neural arch of the vertebra. 



On the fifth and sixth days in 

 the chick, according to Foster 

 and Balfour, the white columns 

 increase rapidly in size, and the 

 anterior median fissure begins to 

 be formed between the anterior 

 columns by their swelling out- 

 wards and leaving its interval 

 between them. It is at first wide and shallo^v, and soon receives a 

 lining of vascular connective tissue or pia mater. The commissures 

 are now also formed ; the anterior grey commissure first, then the pos- 

 terior grey, and somewhat later the anterior white commissure. 



In the further increase of the anterior and lateral white columns as 

 they thicken, they become more united together on each side, so that 

 they can only be arbitrarily distinguished ; the fibres of the roots of 

 the nerves are traced through them into the grey matter ; the cornua 

 of grey matter become more and more developed, and the fissures between 

 the white columns deepen, while the connective tissue or pia-matral 

 septa run more completely inwards through the white substance. 



Angular cells with radiating processes make their appearance in the 

 grey matter, and the nerve-fibres both of the grey and white matter 

 become more distinct. 



The cylindrical cells lining the central canal retain their distinctness, 

 and they are more completely separated from the grey matter by the 

 delicate tissue of the ependyma. Throughout the greater part of the 

 spinal marrow the dorsal part of the primary medullary hollow is 

 obliterated to form the fissure, but in the sacral region of birds it 

 opens out in the rhomboidal sinus, and in the filum tcrminale of the 

 human spinal marrow the whole primary medullary cavity remains. 



The SPINAL COED has been found by Kolliker already in the form of a cylinder 

 in the cervical region of an embiyo four weeks old. Un-united borders have been 

 seen by Tiedemann in the ninth week towards the lower end of the cord, the 

 perfect closing of the furrow being delayed in that part, which is slightly 



