63 THE FIRST CEREBRAL VESICLE. 



tube remains single, and appears as the long stalk partly hollow, and partly solid 

 of a rudimentary median or parietal eye, which occupies an aperture in the middle 

 line of the skull (de Graaf, Baldwin Spencer). The pituitary body (hypophysis 

 cerebri) is chiefly formed by a diverticulum of the buccal epiblast (diverticulum of 

 Rathke) which grows upwards towards the base of the second cerebral vesicle, and 

 dilates into a flask-shaped expansion which is at first simple, but subsequently grows 

 out to form a small mass of epithelial tubes, the lumen of which becomes eventually 

 obliterated. Against the posterior wall of this flask -shaped dilatation the infun- 

 dibulum grows down from the floor of the second vesicle, and its extremity becomes 



ch 



Fig. 76. MEDIAN SAGITTAL SECTION OP THE HEAD IN EARLY EMBRYOES OF THE RABBIT. Magnified. 



(From Mihalkovics.) 



A. From an embryo five millimetres long. 



B. From an embryo six millimetres long. 



In A, the faucial opening is still closed ; in B, the septum is perforated at f ; c, anterior cerebral 

 vesicle ; me, mesencephalon ; mo, medulla oblongata ; m, medullary epiblast ; if (in B), infundibulum ; 

 tspc, spheno-ethmoidal, be, sphenoidal, and spo, spheno-occipital parts of the basis cranii ; i, foregut ; 

 ch, notochord ; py, buccal pituitary involution ; am, amnion ; h, heart. 



Fig. 77. MEDIAN SAGITTAL SECTION OF THE INFUNDIBULUM AND PITUITARY DIVERTICULUM IN A 



RABBIT EMBRYO, AFTER THE OPENING OF THE FAUCES. (From Mihalkovics. ) 



6c, basis cranii with basilar artery ; if, infundibulum ; tha, floor of thalamencephalon ; py, pituitary 

 diverticulum, now closed ; p', stalk of original communication with the mouth ; ph, pharynx ; 

 ch, notochord in the spheno-occipital part of the cranial basis. 



intimately connected with the dilatation, but without communicating with its 

 cavity, although bound up together by the same vascular connective tissue. In 

 connection with this extension of the infundibulum, nerve cells and fibres become 

 formed ; in lower vertebrates they persist and retain their connection with the 

 brain. The notochord extends in the basis cranii as far as the pituitary body. Just 

 before reaching this, it bends ventral wards towards Rathke's diverticulum, and here 

 blends with the buccal epiblast (Bonnet). 



Dohrn has shown that in Petromyzon the hypophysis developes as a separate median 

 diverticulum of the external epiblast which is formed between the nasal pit in front and the 

 buccal invagination (stomodaeum) behind, and grows straight backwards as a canal of some 

 length towards the point of the notochord. where follicles develope from it and become con- 

 nected with the infuniibulum. Later, its orifice is found to open in common with that of the 

 nasal pit. 



The first cerebral vesicle : prosencephalon. This is represented by the 

 common point at the front of the third ventricle, whence the hemisphere vesicles 

 diverge through the foramina of Monro, and by these vesicles themselves. The 

 original vesicle is therefore relatively small, although its lateral outgrowths form by 



