DEVELOPMENT OF ELASMOBRANCH FISHES. 177 



there is present a long opening into the infundibiihim (in), which 

 is shewn to be very narrov/ by being no longer present in fig. la 

 representing a section slightly to one side of the middle line. 

 During the same stage the pineal gland grows into a sack -like 

 body, springing from the roof of the thalamencephalon, fig. Ihypn. 

 This latter (the thalamencephalon) is now dorsally separated 

 from the cerebral rudiment by a deep constrictioD, an J also 

 ventrally by a less well marked constriction. At its side also a 

 deep constriction is being formed in it, immediately behind 

 the pineal gland. The cerebral rudiment is still quite un- 

 paired and exhibits no sign of becoming constricted into two 

 lobes. 



During the next two stages the changes in the fore-brain 

 are of no great importance, and I pas& at once to stage 0. 

 The infundibulum is now nearly in the same condition as 

 during stage L, though (as is well shewn in the figure of a 

 longitudinal section of the next stage) it points more directly 

 backwards than before. The remaining parts of the thalamen- 

 cephalo-n have however undergone considerable changes. The 

 more important of these are illustrated by a section of stage O, 

 PI. XV. fig. 3, trans ver&e to the long axis of the embryo, and 

 therefore, owing to the cranial flexure, cutting the thalamen- 

 cephalon longitudinally and horizontally ; and for stage P in a 

 longitudinal and vertical section through the brain (PI. xv. 

 fig. 5). In the first place the roof of the thalamencephalon has 

 become very much shortened by the approximation of the cere- 

 bral rudiment to the mid-brain. The pineal sack has also 

 become greatly elongated, and its somewhat dilated extremity 

 is situated between the cerebral rudiment and the external 

 skin. It opens into the hind end of the third ventricle, and its 

 posterior w^all is continuous with the front wall of the mid-brain. 

 The sides of the thalamencephalon have become much thick- 

 ened, and form distinct optic thalami {op.) united by a very well 

 marked posterior commissure {p c). The anterior wall of the 

 thalamencephalon as well as its roof are very thin. The optic 

 nerves have become by stage O quite solid except at their 

 roots, into which the ventricles of the fore-brain are for a short 

 distance prolonged. This solidification is arrived at, so far as I 

 have determined, without the intervention of a fold. The 



