BRAIN 203 



to one another and the nature of certain accessory vesicles 

 in this region found in certain forms (e.g. Anguis). Both pineal 

 and parietal organs are in the embryo connected with the brain by 

 a special nerve or tract which grows out from the organ and 

 becomes connected with the brain secondarily (cf. Figs. 165 and 

 168). 



As already stated, a nervous and an epithelial portion are to be 

 distinguished in the hypophysis, the former originating from the 

 infundibulum, the latter from the epithelium of the stomodteum 

 (Fig. 151). In Cyclostomes the nervous portion consists of a thin- 

 walled sac arising from the infundibulum (infundibular sac or 

 process), which in all the true Fishes is in part thrown into folds 

 by the invasion of numerous vessels. Thus arises the so-called 

 saccus vasculosus, the development of which shows great variation 

 amongst the different groups of Fishes. In the higher Vertebrates 

 the infundibular process undergoes various modifications, espe- 

 cially as regards the saccus vasculosus, 1 only traces of which may 

 still be recognisable (e.g. Mammals). 



Both the primary and the secondary fore-brain are situated in 

 the prechordal region of the skull, all the other divisions of the 

 brain lying in its chordal portion (p. 75). The mid-brain and 

 medulla oblongata undergo fewer modifications than the fore-brain, 

 though each optic lobe becomes subdivided into an anterior and a 

 posterior lobe in Mammals ; only the anterior part of the thin roof 

 of the medulla (valve of Vieussens) is nervous, while its floor 

 becomes greatly thickened, and in Mammals gives rise anteriorly 

 to a transverse band of fibres (pons Varolii). It is important to 

 note that the greater number of the cerebral nerves arise from the 

 medulla oblongata. The cerebellum may be more or less distinctly 

 folded and subdivided into median and lateral lobes. 



In the course of further development, the walls of the cerebral 

 vesicles become more and more thickened, so that their cavities, 

 transformed into the ventricles of the brain, undergo a gradual 

 reduction (Fig. 152). 



A series of unpaired ventricles (tdocoele, diaccele, mesoccde, 

 metaccele, and myeloccde) lying in the longitudinal axis of the brain, 

 as well as paired ventricles, can be distinguished. When cerebral 

 hemispheres are more or less distinctly developed, the teloccele 

 gives rise to paired cavities extending into them and known as the 

 lateral ventricles (= ventricles 1 and 2); each of these communi- 

 cates with the diacoele or third ventricle (which extends into the 



1 Various hypotheses have been put forward with regard to the primary 

 nature of the hypophysis : it may represent a sensory organ, or may correspond 

 to the primitive mouth ("palseostoma") of the Protovertebrata, which is to a 

 greater or less extent represented by the combined unpaired nasal and pituitary 

 passage of Cyclostomes (see under Olfactory Organ, and Fig. 190) : on the latter 

 hypothesis the mouth of existing Vertebrates is a "neostoma." It is very 

 probable, especially in the higher Vertebrates, that the epithelial part of the 

 hypophysis lias an important function as a ductless gland, which gives off its 

 secretion into neighbouring blood- and lymph-capillaries. 



