42 W. B. SCOTT AND HENRY F. OSBORN. 



tion of the medullary plate the remainder of the epiblast 

 shows no especial change from the condition seen in the 

 preceding stage. In the medullary plate, on each side of the 

 middle line, is a low rounded ridge (Plate IV, fig. 5), which 

 is formed by the increase in length of the epiblast cells, and 

 perhaps partly also by the wedging in of the mesoblast along 

 these two lines. 



The condition of the spinal cord at this period recalls the 

 the condition of the same organ in the Batrachia of this 

 age. For in the latter the nervous and epidermic layers 

 fuse together into one indiscriminate mass, and do not 

 separate again till much later. This separation takes place 

 for the first time in Triton, not far from the age im which 

 it reappears in the Batrachia. During Stage c sudden and 

 rapid changes make their appearance. The medullary folds 

 are now very prominent, and are composed of numerous 

 elongated spindle- and wedge-shaped cells, while in many 

 places the medullary plate shows a commencement of 

 the same process (Plate IV, fig. 6). But as yet in 

 neither of these regions are any distinct layers to be seen. 

 The lateral epiblast is just beginning to separate into two 

 layers ; the process commences immediately outside of the 

 medullary folds, and spreads down the sides of the embryo, 

 until it has been completed all around (fig. 6). Plate V, 

 fig. 9, shows a drawing on a larger scale of the point where 

 such changes are going on most actively. Even with the 

 aid of this we have not thoroughly satisfied ourselves as 

 to the exact manner in which these changes are accomplished. 

 Three suppositions may be made with regard to it — (1) that 

 the upper layer splits ofif from the lower by a process of 

 cell division ; (2) that the wedge-shaped cells draw in their 

 edges, and lying in alternate arrangement come to make two 

 rows, one above the other ; (3) that both of these have their 

 share in the process. On the whole we rather incline to the 

 latter opinion. In favour of the alternate decrement of 

 length is the fact that for some time preceding the separa- 

 tion the nuclei of the cells are arranged in two alternate 

 rows, very much as in the Elasmobranchs, while such an 

 appearance as shown at the point a, fig. 9, looks as if it 

 could only be cell division. 



Turning to Stage d (Plate IV, fig. 7), we find that in 

 the trunk region the medullary canal is completely closed, 

 and the division of the epiblast carried entirely around the 

 embryo, giving us two well-marked layers. These are com- 

 posed of quadrate, somewhat flattened cells, of nearly equal 

 size in both layers. The cells composing the spinal cord 



