HOMOLOGY. 



11 



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

 the cavity, of the so-called "third ventricle" (5), is now the 

 largest of the embryonal cerebral vesicles ; and this dispro- 

 portion moreover coincides with an incomplete phase of the 

 Vertebrate alimentary canal ; and, what is more to the pre- 

 sent contention, the huge homologue of the " third ventricle " 

 extends into two productions of its wall — one downward ( 8 ) 

 to a canal, " infundibulum," now communicating with the part 

 in contact with the anterior end of the digestive cavity ( 9 ) ; 

 the other upward (7) to the tubular or sacciform condition 

 of the later pineal body*. 



I next pass to the phenomena of the development of the 

 digestive cavity. What subsequently becomes an alimentary 

 canal, begins like the myelon, as a groove, parallel therewith, 

 but opening in the opposite direction, or " hsemad," and there 

 communicating 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: an outlet first 

 appears at the end, which led its discoverer to term it 

 " anus," now known as the " blastopore ; " but, as the canal 

 elongates, it becomes closed at both ends. It absorbs, or 

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



* In his exemplary monograph ' On the Development of Elasmobranch 

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

 bulum becomes much produced, and forms a wide sack in contact with 

 the pituitary hody, 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-like body " (p. 177). . . . 

 At a later stage (P) — " The pineal sack has also become greatly elongated, 

 and its somewhat dilated extremity is situated between the cerebral rudi- 

 ment and the external skin. It opens into the hind end of the third 

 ventricle, and its posterior wall is continuous with the front wall of the 

 mid-brain " (p. 177). 



