182 THE DEVELOPMEXT OF THE TADPOLE. 



arrangement extends throughout the whole spinal cord, but the 

 two portions approximate nearer and nearer as they reach the tail, 

 till at last, merging with each other, they form one black line of 

 cells, showing only the double nature of the cord, but not showing 

 any actual division, as is manifestly the case in the anterior 

 portion. 



By the end of March the first distinct indications of the 

 spinal cord, consisting of two kinds of material — white and grey 

 — can be seen ; the mass of the cord is still intensely black, but 

 is surrounded, especially in the neighbourhood of the medulla, by 

 a mass of lighter cells cohering fairly well together. This 

 adhesion to each other and distinct separation from the remaining 

 portion of the cord was well seen in a section, mounted in rather 

 fluid balsam. After the edge was well set the interior remained 

 fluid, but on the gentlest pressure the external white of the cord 

 separated from the inside darker portion, and regained its original 

 position on relaxing the pressure. Vertical sections now show a 

 large front lobe, not unlike a hammer-head in shape, and the 

 under portion protruding downwards, almost as far as the mouth- 

 cavity in the form of a large crescent, the anterior horn of which 

 is far more curved than the posterior. For some days the only 

 noticeable change which occurs is in the formation of a most 

 delicate membrane which envelopes the whole structure, and 

 evidently represents one or more of the tissues, termed the Arach- 

 noid and Pia Mater. 



On April 5th long threads of nerve substance could be seen 

 anastomosing with each other, and evidently formed by the 

 coalition of several cells. Some of these leave the brain proper 

 to form the nerves. It is most difficult to identify in a section 

 any particular nerve, but a very prominent one — I believe the 

 pneumogastric — is very easily made out. Numbers of multipolar 

 ganglion cells also occur about this time, but can only be seen in 

 very thin parts owing to the exceedingly dark character of the 

 whole. They are especially abundant or can be best seen near 

 the hollow spaces which are developing into ventricles. 



About the middle of April the whole nerve system is in a most 

 interesting transition stage, the fore, mid, and hind brains being 

 well formed, but not arrived at their ultimate development, and the 



