THE DEVELOPMENT OF THE TADPOLE. 183 



two structures, consisting of white and grey matter, are extremely 

 well marked out in horizontal sections. Leaving the fibrous 

 sheath out of the question, the external coat of the spinal cord now 

 consists of cells separated from each other by fairly parallel lines 

 of dark material running through the whole substance and 

 terminating externally in small, dark knobs. In the spinal cord 

 these form two distinct margins, together making about one-half 

 the cord, but gradually coalescing they entirely cover the medulla. 

 The space between is most distinct and occupied by small round 

 nucleated cells entirely free from the dark lines mentioned above. 

 In the tail region the cord thus appears in vertical section to be 

 divided into three parts — the two external ones dark and the 

 internal light. 



The pineal gland can now also be well traced, and, indeed, its 

 complete structure made out. Reserving this for another time, I 

 will only now give an outline as to its position and parts. Situate 

 immediately behind the lobe of the cerebellum and over the 

 medulla oblongata it forms a distinct depression in the latter, to 

 which, however, there is no corresponding external depression, the 

 eye or gland filling up the space. 



Bounded externally by the light ectoderm — as indeed are all 

 other parts of the animal — and resting immediately below the 

 dark epidermis, it forms a large and well marked microscopic 

 object. It may be described roughly as an elongated oval sac, 

 having a transparent, reticulated, rounded portion in front just 

 below the skin ; below this again, and also enveloping it, is a fine 

 granular mass, which apparently corresponds to the nigrum 

 pigmentum of the real eye, but is not nearly so dense ; and below 

 this again is a follicular mass. By this term, " follicular mass," it 

 must not be thought that I mean this portion to have the functions 

 of a follicle, but in its general branched character it possesses far 

 more of the external characters of a follicle than of a retina. 

 This mass extends downwards towards the medulla, and is 

 extended both anteriorly and posteriorly, being in both directions 

 connected with a nerve, the anterior portion of which is attached 

 to the cerebellum and the posterior to the medulla, the connection 

 with the dark external coating of this being very well marked. 

 From a single cord it gradually spreads out, embracing a goodly 



