92 THE THIRD DAY. [CHAP. 



Subsequently however (Per Bau des menschlichen Gehirns) he supposed that 

 it was fi)rined from the pia mater. 



Rathke also subsequently {Entwichlunsgeschichte der Wirhelthiere, Leipzig, 

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

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

 the mesoblast in front of the clinoid process. 



Wilhelm Muller {Ueber die EiitwiMung und Bau der Hi/pophysls und des 

 Processus infu»dibidi cerebri. Jenaische Zeilschrift, Bd. vt. 1871) has recently 

 written an elaborate memoir on the development 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 tlie pituitary body, we must rem.ember that at 

 first the hypoblast of the throat closely underlies the notocliord, 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 h\poblast is as it were constricted off from the alimen- 

 tary canal. In this way tht-re in formed a diveriiculuni 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 constrictintf 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, cndini; 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 

 clinoid process. The base of the vesicle of the third ventride 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, thouErh still bent down- 

 wards, recedes a little from tlie 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 diverticidum 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 

 trabeculee. 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 sta^je becone 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 betvveen the cavity of the diverticulum and that of the throat has become 

 still narrower. The diverticidum 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, wliioli approaches, but does not unite with the pituitary vesicle. 

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



