960 OF GENERATION EMBRYONIC DEVELOPMENT. 



of a canal surrounded by cells arranged in a radiating manner. These 

 separate into an external layer composed of gray matter, and into an inner 

 layer constituting the epithelium of the canal. The white substance appears 

 later than the gray, and is unquestionably developed from it. In the first 

 instance it consists of four cords or strands arranged in pairs, which subse- 

 quently become connected in front by a white commissure. 1 According to 

 the researches of Lockhart Clarke, in Man, Mammalia, and Birds, 2 the 

 spinal cord, in its earlier stages of development, consists of a canal sur- 

 rounded by a homogeneous layer of small cells or nuclei, which are not dis- 

 tinguishable from each other in appearance, and are so closely aggregated 

 as to seem in actual contact. This layer continues to increase in depth, and 

 undergoes a differentiation into two an inner or epithelial, and an outer or 

 gray, layer; while at the same time the small cells of both layers are unin- 

 terruptedly connected by a continuous network of fibres which forms between 

 them. As development progresses, a diversity of structure ensues in the 

 gray substance ; the cells or nuclei of the anterior gray substance becoming 

 much larger than those of the posterior, and being connected by a coarser 

 and more granular network. From these nuclei are developed a number of 

 large roundish or irregular but adjacent cells, with thick nucleated walls, 

 which are connected with the surrounding network, and have precisely the 

 same appearance as the connective tissue of parts external to the cord. It 

 appears, then, that in these early stages of development, there are at least 

 two kinds of free nuclei in the gray substance of the cord. One of these 

 kinds appears to develop the general network of tissue which pervades the 

 entire structure, but proceeds no further; whereas each nucleus of the other 

 kind, while connected with this network as well as with nerve-fibres, de- 

 velops a nucleated cell, with a nucleated wall, which is still connected, and 

 ultimately blended, with the surrounding reticular structure. Through the 

 medium of -this intervening reticular structure, the walls of the nerve-cells, 

 the walls of the bloodvessels, the processes of the epithelium, and the pia 

 mater on the surface of the cord, are all uninterruptedly continuous with 

 each other; and since the processes of nerve-cells constitute the axis-cylin- 

 ders of the vaso-motor nerves distributed to parts external to the cord, Mr. 

 Clarke thinks it probable that some of those processes which are lost by sub- 

 division in the intervening nucleated and reticular tissue within the cord, 

 are the means of transmitting nerve power to that tissue, and through that, 

 to the coats of its bloodvessels, from which, by their direct connection with 

 them, the nerve-cells, in return, receive their supply of nutriment. 



7D8. In the Bird the medullary folds become closed in front, during the 

 early part of the second day, the posterior part remaining open for a short 

 time, as the "sinus rhomboidalis." The anterior extremity is dilated into a 

 bulb, and forms the first cerebral vesicle. Two projections are formed at the 

 sides which gradually become stalked, and constitute the optic vesicles. At 

 the close of the second day the medullary canal presents two more dilata- 

 tions, separated from each other and from the first cerebral vesicle by con- 

 strictions, so that instead of a single cerebral vesicle there ai m e now, in addi- 

 tion to the optic vesicles, a series of them one behind the other. The most 

 anterior of these, or first cerebral vesicle, is developed into the fore-brain, 

 the second into the mid-brain, and the third into the hind-brain. Before 

 the end of the second day the first cerebral vesicle has begun to bud oft' two 

 small vesicles in front; these are the vesicles of the cerebral hemispheres, 

 which subsequently become the most conspicuous part of the brain. As 

 these are formed the fore-brain, with the cerebral and optic vesicles, becomes 



1 Kollikcr, Entwick., pp. 259, 260. 2 Phil. Trans., 1802, p. 911 et ?eq. 



