THE DEVELOPMENT OF THE TADPOLE. 185 



enlargements, like that previously described as surrounding the 

 optic lobes and forming the optic nerve. Around this cord are 

 congregated masses of comparatively large, rounded, nucleated 

 cells, arranged in racemose groups ; and branching off from these 

 we have a number of fibres proceeding outwards on both sides, 

 and passing through the boundary of the medulla to other 

 portions of the system. Ten pairs of these can be readily 

 counted, running in somewhat irregular lines from the centre to 

 the outside of the medulla. Each fibre is in structure a repeti- 

 tion of the central mass, being composed of a chain of elongated 

 nerve-cells, surrounded by round, nucleated cells. The round 

 cells surrounding the central chain are not all of an identical 

 nature, as the more central ones forming a distinct racemose 

 layer stain readily with iodine, whilst those of the branches stain 

 better with picric acid. 



Careful focussing with high powers shows me that, whereas in 

 the posterior part of the ventricle the nerve-chain of the branches 

 is in close proximity to the mass of rounded cells running beneath 

 it, this is not the case with the first four front branches, which 

 on the whole point in a forward direction, whilst the remainder 

 point backwards. These consist of a double nerve-chain placed 

 on a distinctly higher level than the mass of rounded cells seen at 

 the same time. This double chain branches before it leaves the 

 medulla, and in the case of the third from the front each portion 

 of the chain branches, so that, instead of there being two parallel 

 there are four diverging chains of nerve-matter. 



To complete the unity of the whole and the connection 

 between this and the rest of the brain, very small fibrils are given 

 off from each portion of the chain, which, looping into each 

 other, form a chain surrounding the whole, and this is extended 

 in front to a ganglion on either side, where it is connected with 

 other chains of nerve-matter proceeding from the exterior of the 

 cerebrum and optic lobes. Diagrams of these observations can 

 be seen in Figs. 4 and 5, Plate XVII. 



From the very earliest period the nerve-cells, hitherto des- 

 cribed, have been associated with others, which from simple cells 

 have ultimately developed into muscle. Some of these are seen 

 in Fig. I, Plate XVI I. The muscular system is divided into two 



Journal of Microscopy and Natural Science. (1 



New Series. Vol. U. 1889. 



