(330) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." _Mém. Acad. Pétersbourg_, Series VIII., Vol. XVI. 1871. (331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Flusskrebses." _Zeit. f. wiss. Zool._, Vol. XXIX. 1877. THE CENTRAL NERVOUS SYSTEM OF THE VERTEBRATA[158]. [158] 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. The formation of the cerebrospinal axis of the Chordata from the medullary plate has already been treated at length (pp. 301-304). 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[159]. 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. [159] Löwe (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. The relative time at which each of these parts becomes developed is not constant in the different forms. [FIG. 246. SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS' CHICK. _pcw._ dorsal white column; _lcw._ lateral white column; _acw._ ventral white column; _c._ dorsal tissue filling up the part where the dorsal fissure will be formed; _pc._ dorsal grey cornu; _ac._ anterior grey cornu; _ep._ epithelial cells; _agc._ anterior commissure; _pf._ dorsal part of spinal canal; _spc._ ventral part of spinal canal; _af._ anterior fissure.]