92 THE THIRD DAY. [CHAP. 



Subsequently however [Der Ban des menschlichen Gehirns) he supposed that 

 it was formed from the pia mater. 



Rathke also subsequently (EntwicMunsgeschichte der Wirbelthiere, Leipzig, 

 1861) gave up his former view, and believed that the diverticulum of the 

 alimentary canal disappeared, but that the pituitary body was formed from 

 the mesoblast in front of the clinoid process. 



Wilhelm Miiller (Ueber die Entwicklung und Bau der II apophysis und des 

 Processus infundibuli cerebri. Jenaische Zeitschrift, Bd. vi. 1871) has recently 

 written an elaborate memoir on the deve opment and anatomy of the pituitary 

 body and infundibulum in all the orders of Vertebrates, of which the following 

 is an abstract. 



In order to understand the formation of the diverticulum from the ali- 

 mentary canal which forms the pituitary body, we must remember that at 

 first the hypoblast of the throat closely underlies the notochord, and beyond 

 the end of the notochord is almost in contact with the base of the vesicle 'of the 

 third ventricle. When the cranial flexure occurs, which it will be remembered 

 takes place about an axis coinciding with the end of the notochord, the 

 hypoblast, which closely underlies the base of the brain, becomes at the same 

 time bent; and as the angle of the flexure becomes an acute angle, a wedge- 

 shaped space lined by hypoblast is as it were constricted off' from the alimen- 

 tary canal. In this way there is formed a diveniculum of hypoblast which 

 passes forwards from the alimentary canal to the base of the fore-brain — 

 a diverticulum not produced by a forward growth from the alimentary canal, 

 but solely due to the cranial flexure constricting off a wedge-shaped portion 

 of the alimentary canal. This we may call the pituitary diverticulum. When 

 the cranial flexure commences the end of the notochord becomes bent down- 

 ward, and, ending in a somewhat enlarged extremity, comes in contact with the 

 termination of the pituitary diverticulum. The mesoblast around and at the 

 front of the end of the notochord increases and grows up, in front of the 

 notochord and behind the vesicle of the third ventricle, to form the posterior 

 clmoid process. The base of the vesicle of the third ventricle at the same 

 time grows downwards towards the pituitary diverticulum and forms what is 

 known as the infundibulum. This state of things may be observed on the 

 third day. On the fourth day the mesoblast tissue around the notochord 

 increases in quantity, and the end of the notochord, though still bent down- 

 wards, recedes a little from the termination of the pituitary diverticulum, which 

 is still a triangular space with a wide opening into the alimentary canal. 



On the fifth day, the opening of the pituitary diverticulum into the 

 alimentary canal has become narrowed, and around the whole diverticulum a 

 formation of mesoblast-cells has commenced. Behind it the clinoid process 

 has become cartilaginous, while to the side and in front it is enclosed by the 

 trabecular. At this stage, in fact, we have a diverticulum from the alimentary 

 canal passing through the base of skull to the infundibulum. The end of the 

 notochord has at this stai^e become atrophied, so that it is separated by a 

 considerable interval from the pituitary body. 



On the seventh day the mesoblast around the pituitary diverticulum has 

 grown into a complete investment of spindle-shaped cells, and the communi- 

 cation between the cavity of the diverticulum and that of the throat has become 

 still narrower. The diverticulum is all but converted into a vesicle, and its hypo- 

 blast walls have commenced to send out into the mesoblastic investment solid 

 processes, which form the first commencement of the true pituitary body. The 

 infundibulum now appears as a narrow process from the base of the vesicle of 

 the third ventricle, which approaches, but does not unite with the pituitary vesicle. 

 This latter lies in the space between the basi- and the presphenoid, and is 



