THE BRA IN. 



133 



infundibulum (/), formed as a funnel-like extension of the floor, 

 together with a part (the posterior lobe) of the pituitary body 

 (hypophysis) (H}. 1 The other part (anterior lobe) of the pituitary 

 body arises by a gradual pinching off of the epithelium of the primary 

 oral involution, and gives rise later to a gland-like structure. 



The cerebellum, in the higher types, becomes differentiated into 

 two lateral portions (lateral lobes), which may again be sub- 

 divided, and a median unpaired portion (superior vermis), which 



FIG. 106. LONGITUDINAL SECTION THROUGH THE SKULL AND BRAIN OF AX 

 (IDEAL) VERTEBRATE EMBRYO. (Iii part after Huxley.) 



DC, basis cranii ; C/i, notocliord ; SD, roof of skull ; NH'-, nasal cavity; VH, 

 secondary fore-brain (prosencephalon), showing the corpus striatum (Cs) at the 

 base, and the olfactory lobe (Olf) anteriorly ; ZH, thalamencephalon (primary 

 fore-brain), which has given rise dorsally to the pineal gland (epiphysis) (Z), 

 and ventrally to the infundibtilum (/), to which the pituitary body (hypophysis) 

 (H) is attached : anterior to this is seen the optic nerve (Op<), arising from the 

 optic thalanms ( Tito] ; HC, posterior commissure ; MH, mid-brain (mesen- 

 cephalon) ; HH, cerebellum (meteucephalon, secondary hind-brain); NH, 

 primary hind-brain (myelencephalon) ; C?, central canal of spinal cord. 



connects these two. The other two portions of the brain (mid-brain 

 and medulla oblongata) do not become so greatly modified as 

 the fore-brain. It is therefore only necessary to mention that the 

 medulla oblongata, the roof of which undergoes a retrogressive 

 metamorphosis, gives origin to the greater number of the cranial 

 nerves, so that its physiological importance is very great. 



The following important changes take place in the further 

 development of the brain. 



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 constriction. 



1 Opinions are much divided as to the meaning of the epiphysis and hypophysis. 

 Some observers consider the epiphysis to be the homologue of the anterior neuropore. 

 of embryos of Ampliioxus and Tunicata, lhat is, as the remains of a last connection 

 of the brain with the epiblast : this opinion, however, is not tenable according to 

 numerous later researches, all of which agree that it arises secondarily from the 

 roof of the thalamencephalon. Others imagine it to be the last rudiment of a 

 sense-organ, and from the fact that it arises in a similar manner to the optic 

 vesicles it has been explained as an unpaired eye. 



The hypophysis, as it seems to arise as a paired structure, is thought by 

 Dohrn to correspond to a pre-oral gill-cleft : this is the latest of the many hypotheses 

 which have been put forward on the subject. 



