402 W. H. GASKELL. 



spinal cord a system of supporting elements derived from the 

 walls of the old intestine, as I have suggested in my former 

 papers, may be formed after the fashion described by His. 

 At the same time I agree with Ahlborn that we cannot look 

 upon the epithelial layer of the central canal as many-layered 

 until the origin of these small cells has been demonstrated 

 embryologically. Certainly, as far as can be judged from 

 His's (14, fig. 30) figure of a section of the cord (-f a larva of 

 Petromyzon fluviatilis 6 ram. long, the epithelial layer at 

 that age is only one cell thick, and also from his figure and 

 description there is no sign of any supporting tissue elements 

 (myelospongium) outside the layer of epithelium, but only 

 of nervous elements, so that any formation of supporting 

 structures from this epithelium layer must take place at a 

 later stage than in an Ammoccetes of 6 mm. long. In Scott's 

 description of the closure of the canal his figures are drawn 

 from larvae of 7 mm. in length and upwards. 



Sect. 5. — The Cephalic Liver and its Duct. 



We have already noticed how clearly the fatty degeneration 

 of the columnar epithelium cells which line the cavities of the 

 brain enables us to trace out the original limits of those cavities 

 even when the lumen has been closed by the approximation of 

 their walls, as in the formation of the raphe. In the same way 

 any diverticula from the main cavity will be disclosed by the 

 black staining of the fat-globules in its walls, even if its lumen 

 has entirely disappeared. 



In the medulla oblongata the vertical slit which forms the 

 raphe comes close to the surface on the ventral side (figs. 

 13 «, 14, PI. XXVI); on the other hand, in the region of the 

 aqueduct, as seen in fig. 3, PI. XXV, the raphe is short, and 

 only extends a short distance on the ventral side. Now the 

 series of sections show that the most ventral portion of the 

 fold which forms the raphe ceases abruptly at the projection 

 of the con us post-commissuralis, its termination coincid- 

 ing with the position of the so-called ganglion interpe- 

 dunculare. 



