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Natural History Bulletin. 



hypophysis cerebri. Here, as elsewhere, ■pittutary body is 

 used for hyphophysis cerebri. As a thorough knowledge of 

 the opinions of previous observers is necessary to an under- 

 standing of this paper, it is thought best to introduce the 

 entire review. 



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

 controversy amongst embryologists. Von Baer (loc. cit.) and Smidt {Zeitschrift filer Wiss. Zoolo- 

 gie, 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 " infundibulum," which subsequently became 

 expanded at its termination to form the pituitary body, 



Rathke [Archiv fuer Anatomie unci Physiologie, 1838, Bd. v.) states that very early a diverticu- 

 lum is produced from the upper end of the alimentary canal, which passes backwards and meets 

 the process of the brain called the infundibulum. This diverticulum subsequently loses all con- 

 nection with the epithelium of the digestive canal, and. uniting with the infundibulum, forms the 

 pituitary body. 



Dursy {Eiztwicklungsgeschichte des Kopfes, Tiibingen, i85g) states that both the end of the 

 notochord and the epithelium of the alimentary canal take part in the formation of the pituitary 

 body. The apparent diverticulum of the alimentary canal is not so much a true diverticulum, as a 

 part of the alimentary canal constricted off from the remainder by the cranial flexure. 



Reichert {Entwicklungsleben im Wirbelthierreich, Berlin, 1840) states that the pituitary body 

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



Subsequently however (Der Bau des mcnschlichen Gehirns) he supposed that it was formed 

 from the pia mater. 



Rathke also sub'^equently [Ent'<vicklungss;eschichte 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 (Weber die Entwicklung und Bau der Hypophysis und des Processus infun- 

 dibuli cerebri, yenaische Zeitschrift, Bd. vi. 1871) has recently written an elaborate memoir on 

 the development and anatomy of the pituitary body and infundibulum in all the orders of Verte- 

 brates, of which the following is an abstract. 



In order to understand the formation of the diverticulum from the alimentary 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 fle.vure 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 Sdme 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 alimentary 

 canal. In this way there is formed a diverticulum 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 downward, 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 clinoid process. 

 The base of the vesicle of the third ventricle at the same time grows downwards towards the pitui- 

 tary 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 th^ end of the notochord, though still bent downwards, 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 trabecule. 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 stage become 

 atrophied, so that it is separated by a considerable interval from the pituitary body. 



