v.] THE FORE-BRAIN". 91 



on the second day began to grow out from the front of the 

 fore-brain, increase rapidly in size during the third day, so 

 much so that by the end of the day each of them (Fig. 24, CH, 

 Fig. 25, la) is as large or larger than the original fore-brain 

 from which they both sprang, and they form together a most 

 conspicuous part of the brain. In their growth they push 

 aside the optic vesicles, and thus contribute largely to the 

 roundness which the head is now acquiring. Each vesicle 

 possesses a cavity, known afterwards as a lateral ventricle, 

 which, though quite separate from its fellow, is continuous 

 with the cavity of the fore-brain. 



Owing to the development of these cerebral hemispheres, 

 the original fore-brain no longer occupies the front position 

 (Fig. 24, FB, Fig. 25, lb), and ceases to be the conspicuous 

 object that it was. Inasmuch as its walls will hereafter be de- 

 veloped into the parts surrounding the so-called third ventricle 

 of the brain, we shall henceforward speak of it as the vesicle 

 of the third ventricle, or, inasmuch as it soon comes to lie 

 between the expanded posterior ends of the cerebral hemi- 

 spheres, as the tween brain. 



On the summit of the 'tween brain there may now be 

 seen a small conical projection, the rudiment of the pineal 

 gland (Fig. 25, e), while the centre of the floor is produced 

 into a funnel-shaped process, the infundihulum (Fig. 22, d), 

 which, stretching towards the extreme end of the alimentary 

 canal, joins the pituitary body. 



The development of the pituitary body or hypophysis cerebri has been the 

 subject of considerable controversy amongst embryologists. Von Baer (loc. cit.) 

 and Smidt {Zeilschrlfl fiir Wiss. Zoologie, 1862, B. xi, p. 43) believed that the 

 base of the fore-brain, or vesicle of the third ventricle, became produced into a 

 downward process, the 'infundihulum,' which subsequently became expanded at 

 its termination to form the pituitary body. 



Rathke (Archiv fiir Anatomieund Physiologic 1838, Bd. V.) states that very 

 early a diverticulum is produced from the upper end of the alimentary canal, which 

 passes backwards and meets the process of the brain called the infundibulnm. 

 This diverticulum subsequently loses all connection with the epithelium of the 

 digestive canal, and, uniting with the infundihulum, forms the pituitary body. 



Dursy (Entwicklangsgeschichte des Kopfes, Tubingen, 1869) states that both 

 the end of the notochord and the epithelium of the ahraentary canal take part 

 in the formation of the pituitary body. The apparent diverticulum of the ali- 

 mentary canal is not so much a true diverticulum, as a part of the alimentary 

 canrtl constricted off from the remainder by the cranial flexure. 



Reichert (EntmcUungsleben im Wirbelthierreich. Berlin, 1840) states that 

 the pituitary body is formed from the remains of the front end of notochord. 



