OF THE CONAEIO-HTPOPHTSIAL XllACT. 137 



In the Vertebrate embyro (fig. 4) the myeleucephalon, i-e, first 

 appears as a longitudinal channel of the ectoblast, opeuioo- 

 "neurad;" and soon, by upward or neural extension and con- 

 vergence of its side-walls, it is converted by their confluence into 

 a tube. 



Passing over the histological steps iu the formation of the grey 

 and white matters and the reduction thereby of the canal to the 

 minute central one of the adult myelon, what here concerns my argu- 

 ment is the progressive forward extension of the cord, with corre- 

 sponding expansions into the beginnings of the "hind brain," or 

 epencephalon (fig. 4, s), of the mid brain or mesencephalon (ih. i), 

 and of a large vesicle {ib. s), dividing the latter from the fore brain, 

 or " prosencephalon" (ib. e). All these expansions, as shown in the 

 diagram (fig. 4), are hollow; but the relative size of the cavity, of the 

 so-called "third ventricle " (s), is now the largest of the embryonal 

 cerebral vesicles ; and this disproportion moreover coincides with 

 an incomplete phase of the Vertebrate alimentary canal ; and, 

 ■what is more to the present contention, the huge homologue of 

 the " third ventricle " extends into two productions of its wall, 

 one downward (s) to a canal, " infundibulum," now communica- 

 ting with the anterior end of the digestive cavity (9) ; the other 

 Upward (7), to the " thalamencephalon " *. 



I next pass to the phenomena of the development of the diges- 

 tive cavity. What subsequently becomes an alimentary canal, 

 begins like the myelon, as a groove, parallel therewith, but open- 

 ing in the opposite direction, or " haemad," and there communi- 

 cating with the vitellicle. It is developed most conspicuously or 

 in greatest proportion from the hypoblast. As the alimentary 

 rudiment extends beyond the yolk-sac, forward and backward, it 

 becomes tubular, but as yet is closed at both ends. It absorbs, or 

 receives, nutriment from the yolk-bag, which recedes as it dimin- 

 ishes and becomes excluded from the abdomen by completion of 



* In his exemplary monograph ' On the DeTelopment of Elasraobranch 

 Fishes,' 8vo, 1878, Mr. Balfour writes : — " During stage L the infundibu- 

 lum becomes much produced, and forms a wide sack in contact with the pitui- 

 tary body, and its cavity communicates with that of the third ventricle by an 



elongated slit-like aperture " (p. 176) " During the same stage the pineal 



gland grows into a sack-hke body " (p. 177) At a later stage (P)— '-The 



pineal sack has also become greatly elongated, and its somewhat dilated ex- 

 tremity is situated between the cerebral rudiment and the external skin. It 

 opens into the hind end of the third ventricle, and its posterior wall is con- 

 tinuous with the front wall of the mid-brain ' (p. 177). 



LnW. JOUBN. — ZOOLOaT, VOL. XVI. 10 



