:U2 SPINAL CORD. 



(330) A. Kowalevsky. "Embryologische Studien an Wiirmern u. Arthro- 

 poden." Mem. Acad. Peterxboimi, Series vin., Vol. xvi. 1871. 



(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Fluss- 

 krebses." Zeit. f. IP ins. Zool., Vol. xxix. 1877. 



THE CENTRAL NERVOUS SYSTEM ot THE VERTEBRATA'. 



The formation of the cerebro-spinal axis of the Chordata from the 

 medullary plate has already been treated at length (pp. 250 252). 

 Before entering into the consideration of the morphological value of 

 the various parts of this cord, it will be convenient to describe the 

 more important features of its ontogeny. For this purpose the two 

 parts into which the nervous axis becomes at an early period divided, 

 viz. the spinal cord and the brain, may be dealt with separately. 



The Spinal Cord, shortly after the closure of the medullary canal, 

 has, in all the true Vertebrata, the form of an oval tube ; the walls of 

 which are of a fairly uniform thickness, and are composed of several rows 

 of elongated cells. This cord, as development proceeds, usually becomes 

 vertically prolonged in transverse section, and the central canal which 

 it contains also becomes vertically elongated. The variations in shape 

 of the spinal canal are very great at different periods and in different 

 parts of the body, and an attempt to chronicle them would appear, in 

 the present state of our knowledge, to be quite valueless". Fig. 117, 

 in which the spinal cord of the chick of the third day is shewn in 

 transverse section, illustrates the character of the cord at the stage just 

 described. Up to this time the walls of the spinal canal have exhibited 

 an uniform structure. A series of changes now however takes place, 

 which results in the differentiation (1) of the epithelium of the central 

 canal, (2) of the grey matter of the cord, and (3) of the external 

 coating of white matter. 



The relative time at which each of these parts becomes developed 

 is not constant in the different forms. 



The white matter is apparently the result of a differentiation of 

 the outermost parts of the superficial cells of the cord into longitudinal 

 nerve-fibres, which remain for a long period without a medullary 

 sheath. These fibres appear in transverse sections as small dots. The 

 white matter foims a transparent investment of the grey matter and 

 would seem to contain neither nuclei nor cells 3 . The white matter 

 may from the first form only two masses, one on each side, forming 



1 For the development of the central nervous system in Amphioxus and the Tunicata 

 the reader is referred to the chapters dealing with those two groups. 



2 Lowe (No. 341) holds that at an early stage of development three regions can 

 always be distinguished in any section of the central canal, viz. (1) a ventral narrow 

 slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often 

 be observed, but my own observations do not lead me to attach any special importance 

 to it. 



3 This holds true at first for Elasmobranchii, but at a later stage there are present 

 numerous nerve-cells in the white matter, so that the distinction between the white 

 and grey matter becomes much less marked than in higher types; in this respect Elas- 

 mobranchii present an approximation to Amphioxus. 



