THE DEVELOPMENT OF THE TADPOLE. 179 



some general outline of the chief changes in form undergone by 

 the brain from the very earliest stages. 



As already mentioned in page 37, Vol. I., the first indications 

 of brain development is seen in the formation of a canal, called 

 the neural canal, formed by the upper end of the egg first 

 becoming flat, then depressed, and ultimately the sides of this 

 depression close together to produce a groove, called the medullary 

 groove. When this medullary groove has become quite closed 

 and thus converted into a perfect canal, the spinal cord is formed 

 within it, first, as an aggregation of very dark nucleated cells, 

 grouped together so as to leave a central vacant space of about 

 the same diameter as a single cell, represented on a small scale in 

 Plate II., Figs, i and 3, of Part I., N.S. These dark cells, though 

 perfectly separate and very distinguishable from the mass of loose 

 tissue, composed of much larger cells surrounding them, do not 

 present any definite grouping amongst themselves, except that 

 they may very roughly be divided into three or four layers. 

 Individual cells, however, of the same size, form, and character 

 occur at some distance from them towards the edge of the 

 segmentation cavity which exists immediately below the principal 

 mass. It is difficult from the intense blackness of these cells to 

 distinguish their real characters, but their general appearance is 

 that of a cell surrounded by a dense cell-wall, and containing a 

 crowd of very black nuclei, as attempted to be delineated in Plate 

 XVIL, Fig. I. 



This hollow rod soon takes the form of an oval tube with a 

 narrow, elongated bore, the sides of which rapidly approach each 

 other about the middle point, but rather nearer the lower than the 

 upper end, and thus the single tube becomes separated into two 

 tubes — the lower one much smaller than the upper. The walls of 

 this tube consist of elongated cells, the external ones — that is, 

 those farthest from the centre — being much darker and far more 

 elongated than the interior ones. Many of the dark cells are 

 branched inwards and terminate on the exterior in small knobs, 

 whilst the branched interior portions are connected with the 

 lighter-coloured cells forming the inside portion of the brain. 

 Although it was not possible to make out the connection in every 

 case, yet in a number of instances it is certain that these two 



