124 THE FROG. 



frog, retaining its tubular character, and its communication 

 with the third ventricle. 



About the time the roof of the fourth ventricle is becoming 

 folded, and the choroid plexus established in connection with it, 

 a similar change is going on in relation with the third ventricle. 

 Immediately in front of the pineal body, the thin roof of the 

 thalamencephalon becomes thrown into folds which hang down 

 into the ventricle (Fig. 65, x). A dense plexus of blood-vessels 

 lies on the roof and grows in between its folds, giving rise to a 

 choroid plexus similar to that of the fourth ventricle, but more 

 restricted in its extent. The vascular plexus on the surface 

 of the thalamencephalon forms also a dorsally projecting process, 

 the supra-plexus, with which the distal end of the stalk of the 

 pineal body is in very close relation (Fig. 89, PN). 



The infundibulum (Figs. G4, 65, IN) is a depression of the 

 floor of the thalamencephalon, with which the pituitary body 

 comes into close relation at an early stage. The infundibulum 

 is already recognisable at the time of closure of the neural tube : 

 its hinder wall is in close relation with the anterior end of the 

 notochord, and it is in fact the infundibular depression (Fig. 61) 

 which causes the brain to appear to be bent round the end of 

 the notochord, one of the most striking features of cranial 

 flexure. The infundibulum is separated from the mid-brain 

 (Figs. 61, 65) by a deep transverse groove, running across the 

 ventral surface of the brain, and very conspicuous when the 

 brain is seen from below. On the appearance of the optic 

 vesicles, a second transverse groove (Fig. 64) is formed, further 

 forwards, between the optic vesicles and the infundibulum. The 

 infundibulum retains the same character and relations through- 

 out all the later stages of development : it appears as a wide 

 thin-walled sac, forming a conspicuous projection on the under 

 surface of the brain, and having the pituitary body in close 

 relation with it posteriori}* (Fig. 89, IN). 



The pituitary body, although not really a part of the brain, 

 may conveniently be described here. It arises (Figs. 60, P, and 

 61, PT) as a plug-like ingrowth of the deeper or nervous layer 

 of the epiblast, immediately below the anterior end of the brain. 

 It appears very early, and may be recognised as a slight thicken- 

 ing of the epiblast even before the neural tube is closed ; and at 



