[147] 



BcvclopiiKut of tbc ^abpole. 



By J. W. Gatehouse, F.I.C. 

 Part III. Plate XIII. 



N our last article mention was made of the various stages of 

 development occurring up to March 21st — that is, about 

 three weeks after the egg was deposited ; in the present 

 paper sections will be given of a single animal as it appeared on 

 March 17th. These are numbered in succession, i, 2, 3, 4, and 

 5, number 3 being as nearly as possible a median section, whilst 

 I, 2, and 4, 5 are two sections on each side of it respectively. 



The first thing which strikes us on looking at these sections is 

 the enormous development of the brain and spinal cord in com- 

 parison with the size of the entire animal. The nervous system 

 fills up the whole upper portion of the cavity of the skull, extend- 

 ing down nearly to the cavity of the mouth, and behind stretching 

 out as a thick cord to the very extremity of the body. When we 

 compare this with the relative size of the brain and spinal cord as 

 existing in the adult frog, it will be at once seen that an immense 

 amount of both concentration and diffusion must occur between 

 the present and the adult stages — concentration in the approxima- 

 tion of the various portions of the nerve system to each other, 

 and a possible diffusion by portions of the nerve matter spreading 

 out so as to produce nerve-cords extending over the body. 



Whether the whole of the nervous system is developed from 

 the nerve-matter at present formed, or whether fresh portions of 

 yelk assist in the formation of nerve substance will probably be 

 seen in our future sections. The length of the brain itself — 

 extending as it does from the very front portion of the head, 

 quite back to the segmented sheath surrounding both the spinal 

 cord and notochord — is much greater in proportion to the whole 

 nervous system in the embryo than it is in the adult. Thus, at 

 the present stage, the cerebral lobes, medulla oblongata, etc., are 

 at least as long as the remainder of the nerve-cord, but in the frog 

 the latter, within the vertebral column, must be quite three times 

 the length of the true brain, even if we include the olfactory lobes 

 as a portion of it. 



