The Development of tlie IVIanimalian Pituitary Body 171 



ance are due to tlie strong growth of connective tissue around it and the 

 development of the sphenoid bone. It is unnecessary to attribute the 

 change to the pressure exerted on the neck of the sac by the carotid arteries 

 and their growing adventitia, as did W. Mliller. The above explanation 

 of Mihalkovics is probably the right one. The hypophysial sac is now 

 of considerable size and extends laterally, its anterior wall being still in 

 close connection with the wall of the cerebral vesicle. A well-marked 

 invagination of the wall of the latter constitutes the infundibular process ; 

 its anterior wall is also closely invested by the epithelium of the wall of 

 the hypophysial sac. The infundibular process becomes the nervous 

 portion of the posterior lobe of the pituitary, while the part of the wall of 

 the hypophysial sac adhering to it constitutes the epithelial covering of 

 the posterior lobe or " Epithelsaum " of Lothringer. Epithelium and 

 nervous tissue have been in close contact with one another from their first 

 appearance. The cavity of Rathke's pouch persists throughout life as the 

 epithelial cleft which partly separates the anterior from the posterior lobe. 

 In its lateral extension the sac is beginning to envelop the sides of the 

 infundibular process. Its walls are composed of cylindrical cells which 

 closely resemble those of the infundibular process. They are thickened in 

 the anterior part of the sac in the region of its neck, where there is a 

 distinct fold ; the thickening in this situation is the beginning of the 

 anterior lobe proper of the pituitary. Its cells are not as yet differentiated 

 from the cells of the remainder of the sac. 



During subsequent development the pituitary body is removed further 

 and further from the nasal mucosa by the growth of the sphenoid bone. 

 Ossification in the latter is delayed for some time by the persistence of a 

 cord of epithelial cells connecting the anterior lobe of the pituitary with the 

 nasal mucous membrane. This connection is still found in cat embryos of 

 from 35 to 40 mm., but is then imperfect and soon after disappears, allowing 

 the opening in the bone to close up. Differentiation between anterior lobe 

 proper and the pars intermedia now begins to take place. The anterior 

 lobe is formed by a proliferation of the cells of the lower part of the 

 anterior wall of the sac just above its neck. Solid columns of cells are 

 formed in this situation, and invade the cavity of the sac so as gradually to 

 fill it, leaving only a narrow space or cleft between them and the epithelium 

 covering the posterior lobe. The anterior lobe also grows forward and 

 laterally. The neck of the sac retains a tubular character for some time, 

 and becomes somewhat convoluted. One of these convolutions (fig. 5, Z) 

 applies itself to the under surface of the brain and gives rise to the tongue- 

 shaped process which extends forwards from the anterior lobe towards the 

 optic chiasma. 



The structures which enter into the formation of the pituitary are 

 closely related to large blood-vessels from their earliest appearance. In 

 figs. 1 and 2 a large blood-channel is seen lying immediately behind 

 Rathke's pouch, in front of the notochord. Dursy, indeed, as already 



