4OO DEVELOPMENT OF ELASMOBRANCH FISHES. 



its cavity communicates with that of the third ventricle by an 

 elongated slit-like aperture. This may be seen by comparing 

 PI. 16, figs. \a and ic. In fig. ic taken along the middle line, 

 there is present a long opening into the infundibulum (/;/), which 

 is shewn to be very narrow by being no longer present in fig. \a 

 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. ib,pn. 

 This latter (the thalamencephalon) is now dorsally separated 

 from the cerebral rudiment by a deep constriction, and 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 unpaired 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 pass at once to stage O. 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 thalamencephalon 

 have however undergone considerable changes. The more im- 

 portant of these are illustrated by a section of stage O, PI. 16, 

 fig. 3, transverse to the long axis of the embryo, and therefore, 

 owing to the cranial flexure, cutting the thalamencephalon 

 longitudinally and horizontally; and for stage P in a longi- 

 tudinal and vertical section through the brain (PI. 16, fig. 5). 

 In the first place the roof of the thalamencephalon has become 

 very much shortened by the approximation of the cerebral 

 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 wall is continuous with the front wall of the mid- 

 brain. The sides of the thalamencephalon have become much 

 thickened, and form distinct optic thalami (op.} united by a very 

 well marked posterior commissure (pc.}. 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 



