ON THE CELLULAR THEOEY OF DEVELOPMENT. 97 



fore, presents this interesting and remarkable pecu- 

 liarity in Scyllium and Acanthias; it grows or is 

 differentiated from the ciliary ganglion to the floor 

 of the mid-brain, and not in the opposite direction, as has 

 hitherto been supposed. The proof of this is to be found in the 

 fact that in a Scyllium and Acanthias embryo of 10 to 

 11 mm. the third nerve can be seen projecting forwards from 

 the ciliary ganglion, and ending in front in the reticulum, 

 short of the floor of the mid-brain. The ciliary or profundus 

 ganglion is at one time — when it is first laid down — in contact 

 with the ectoderm. Later it is shifted inwards, but remains 

 connected for a time with the ectoderm by a cord of cells, 

 which eventually disappears. This point has been seen by 

 van Wijhe. 



The embryonic medullary walP is connected with the 

 reticulum by pale fibres similar to those which compose 

 the reticulum, and the nerve-roots, both anterior, 

 posterior, and cranial, are special enlargements of 

 such connecting strands. They are formed at a time 

 when no structures which could be called cells by any but a 

 fanatical devotee of the cellular theory are present, either in the 

 medullary wall or in the ganglionic rudiments, and in a manner 

 which, if closely followed, renders it quite impossible to speak 

 of growths one way or the other, excepting that one can make 

 one assertion — the pale fibrous substance which marks the 

 nerve appears both in the anterior and posterior roots and in 

 the cranial nerve-roots next the central organ, at a time when 

 the white matter (which is composed of this pale fibrous sub- 

 stance) first appears as a thin layer, and in continuity with 

 such white matter. The differentiation outwards pro- 

 ceeds from this point, and the free end of the nerve- 

 rudiment always ends by branching out into the fibres 



my fuller paper dealing with this subject I hope to examine Dohrn's results in 

 detail. 



^ Inasmuch as the nerve-crest is derived from the medullary wall and gives 

 rise to mesodermal structures, the medullary wall itself gives rise, in part, to 

 mesoderm. 



VOL. 37, PART 1. NEW SER. G 



