90 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



granular material produced by the breaking down of the epithelial 

 lining. 



The paraphysis forms for a time a conspicuous tube passing 

 upwards and forwards in the space between the two hemispheres 

 and ending blindly. In later stages of development it undergoes a 

 relative reduction in size, and becomes irregularly twisted and mixed 

 up with the choroid plexus of the ventricles. 



On either side of the paraphysis and just dorsal and posterior to 

 its base, the wall of the brain becomes involuted into the third 

 ventricle, the involuted portion being thin and membranous and 

 enclosing an ingrowth of blood-vessels. This vascular ingrowth 

 represents a structure which in most Vertebrates is continuous across 

 the mesial plane with its fellow so as to form an unpaired structure 

 the velum transversum. This is regarded by most writers on the 

 brain as an important landmark in brain topography. 



On the floor of the thalamencephalon the optic chiasma and 

 the anterior commissure form prominent bulgings into the ventricle. 

 Each develops nerve-fibres in its substance, connected in the one case 

 with the organs of vision and in the other with the cerebral hemi- 

 spheres, especially those portions devoted to the sense of smell. 



In front of the optic chiasma lies a deep optic recess which is 

 prolonged outwards by an outgrowth of the side wall of the brain, the 

 optic outgrowth, which gives rise to a great part of the eye and 

 will be described later. Behind the chiasma is the infundibulum, 

 the tip of which at a late stage in development (about stage 38) 

 sprouts out into narrow tubular diverticula. These increase in 

 length, wind hither and thither, and partially penetrate into the 

 substance of the pituitary body which lies immediately beneath. 

 The epithelium of these tubular diverticula assumes a glandular 

 appearance and together they constitute the " infundibular gland " — 

 often called the " nervous portion of the pituitary body." 



The series of sagittal sections in Fig. 53 is of interest from its 

 bearing upon a question which has excited some discussion, namely 

 as to what point in the fully developed brain of the vertebrate 

 corresponds to the morphologically anterior end of the brain rudi- 

 ment in earlier stages of development. It has been held by many 

 morphologists, such as von Baer, His, Sedgwick, that the tip of the 

 infundibulum represents the anterior end of the primitive brain, the 

 present condition having been brought about by the anterior portion 

 of the brain becoming bent upon itself into a retort shape. As will 

 be seen by an inspection of the figures the brain of Lepidosiren lends no 

 support to this idea. On the contrary the tip of the infundibulum 

 clearly corresponds to a point close to the letter A of Fig. 53, A. 

 On the other hand, equally clearly the anterior tip (*) of the brain- 

 floor of an early stage such as that shown in Fig. 53, B is represented 

 in the adult by a point well up on the anterior wall of the thalamen- 

 cephalon (lamina terminalis) and just ventral to the root of the 

 paraphysis. 



