The Hypophysis Cerebri. 299 



On the seventh day the mesoblast around the pituitary diverticulum has grown into a complete 

 investment of spindle-shaped cells, and the communication beUveen the cavitj- of the diverticulum 

 and that of the throat has become still narrower. The diverticulum is all but converted into a 

 vesicle, and its h)-poblast walls have commenced to send out into the mesoblastic investment solid 

 processes, which form the first commencement of the true pituitary bodj-. 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 pituitarj- vesicle. The latter lies in the space between the basi- and the 

 pre-sphenoid, and is completely surrounded bj- a ring of cartilage. The mesoblast-cells immediately 

 around it do not, however, exhibit any signs of becoming cartilaginous. 



By the tenth day the opening of the pituitary vesicle into the throat becomes almost obliterated, 

 and the lumen of the vesicle itself very much diminished. The body itself consists of anastomosing 

 cords of hj-poblast-cells, the mesoblast berA-een which has already commenced to become vascular. 

 The cords or masses of hj-poblast cells are surrounded by a delicate membrana propria, and a few ot 

 them possess a small lumen. The infundibulum has increased in length. 



On tJie twelfth day the communication betiveen the pituitar>- vesicle and the throat is entirelj' 

 obliterated, but a solid cord of cells still connects the two. ITie vessels of the pia mater of the 

 vesicle of the third ventricle have become connected with the pituitarj' body and the infundibulum 

 has grown down along its posterior border. 



In the later stages, all connection is lost betiveen the pituitarj' body and the throat, and the 

 former becomes connected with the elongated processus infundibuU. 



Such is Wilhelm Mailer's account. Goette, however, yArchh'. Micr. Anat. IX. p. 397) has 

 recently given reasons for thinking that the pituitary diverticulum arises not from the closed fore- 

 gut, lined with hypoblast, but from the buccal cavitj- lined with epiblast. He states that in its 

 earlier stages it may be seen to start on the oral side of the partition, which for some time di^-ides 

 the secondarily formed buccal ca\-ir\' from the primarily formed foregut, and therefore, belonging 

 to the former, cannot be regarded as the natural anterior termination of the latter. 



Another view is presented by Balfour. ^ "The pituitary 

 body is in fact an organ derived from the epiblast of the 

 stomodteum. This fact has been demonstrated for Mammalia, 

 Aves, Amphibia, and Elasmobranchi, and may be accepted 

 as holding good for all the Craniota." 



In Grav's Anatomy ^ we find a very curious statement of 

 the development of the pituitary body. A footnote on page 

 120, presumiblv bv Prof. John K. Rvder, gives a much 

 clearer and more exact account. " The epiblast of the back 

 part of the roof of the oral invagination, where a hollow, 

 saccular portion of the oral epiblast is constricted off and fuses 

 with the median infundibular process of the floor of the brain, 

 is developed into the pituitarv body or hypophysis. The 

 fusion of the infundibulum with the involution from the epi- 

 blast of the upper and posterior part of the oral cavity leads 

 to the formation of the pituitary body or hypophysis. This, 

 therefore, is entirely of epibiastic origin, and its relations indi- 



^ Comparative Embryology, by F. M. Balfour. Macmillan .S: Co., London 

 iSSo. Vol. II.. page 35S. 



2 Gray's Anatomy. Lea Brothers & Co., Philadelphia. 1SS7. Page 119. 



