NERVOUS SYSTEM OF THE VERTEBRATA. 



3.17 



ptn^ 



anfi 



cartilage of the cranium (Acaiitliias), or be placed outside tlie 

 cranium (Raja). 



In Petromyzon its form is ver}' different. It arises (fig. 253 'pn) 

 as a sack-like diverticulum of the thalamencephalon extending at 

 first both backwards and forwards. In the Ammocoete the walls of 

 this sack are deeply infolded. 



The embryonic form of the pineal gland in Amphibia is very much 

 like that which remains permanent in Elasmobranchii ; the stalk 

 connecting the enlarged terminal portion with the brain soon however 

 becomes solid and very 



thin except at its prox- j^^ 



imal extremity. The Jmr \ 



enlarged portion also ad- 



becomes solid, and is sot "■. ..•■•"r , v «; -^^k^^ / y^^" 



placed in the adult 

 externally to the skull, 

 where it forms a mass 

 originally described by 

 Stieda as the cerebral 

 gland. 



In Birds the primi- 

 tive outgrowth to form 

 the pineal gland be- 

 comes, according to 

 Mihalkovics, deeply 

 indented by vascular 

 connective tissue in- 

 growths, so that it 

 assumes a dendritic 

 structure (fig. 250 /;i??). 



The proximal ex- 

 tremity attached to the 

 roof of the thalamen- 

 cephalon forms a spe- 

 cial section, known as 

 the infra-pineal pro- 

 cess. The central lu- 

 men of the free part 

 of the gland finally 

 atrophies, but the 



cufd 



pdt. 



/1/lS 



Fig. 255. Longitudinal vertical section thkough 

 the anterior part oe the rrain of an embryo rabbit 

 OF FOUR CENTIMETRES. (After Mihalkovics. ) 



The section passes throu.;h the median line so that 

 the cerebral hemispheres are not cut; their position is 

 however indicated in outline. 



spt. septum lucidum formed by the coalescence of 

 the inner walls of part of the cerebral hemispheres ; 

 cna. anterior commissure; frx. vertical pillars of the 

 fornix; cal. genu of corpus callosum; trm. lamina ter- 

 minalis; hms. cerebral hemispheres; olf. olfactory lobes; 

 acl. artery of corpus callosum ; fmr. position of foramen 

 of Munro; did ^. choroid plexus of third ventricle; piiu 

 pineal gland; c'mp. posterior commissure; hgm. lamina 

 uniting the lobes of the mid-brain; chin, optic chiasma; 

 hph. pituitary body; inf. infundibulum ; pns. pons Va- 

 rolii; pde. cerebral peduncles; agd. iter. 



branches still remain 



hollow. The infra-pineal process becomes reduced to a narrow stalk, 

 connecting the branched portion of the body with the brain. The 

 branched terminal portion and the stalk obviously correspond with 

 the vesicle and distal part of the stalk of the types already described. 

 In Mammalia the development of the pineal gland is, according to 

 Mihalkovics, generally similar to that of Birds. The original out- 

 growth becomes branched, but the follicles or lobes to which the 



