176 Herring 



covering the neck and part of the posterior lobe is relatively smaller than 

 it is in the cat embryo. But the intermediate part is not really reduced ; 

 it has changed its position, and in the human embryo is found to extend 

 further over the surface of the brain. A thin layer (fig. 7, h) is prolonged 

 forwards and backwards (6') over the brain substance adjacent to the neck. 

 The cells are arranged in columns, which may be a single cell thick, no 

 lumen being found in them. Blood-vessels accompany this layer, and 

 pass freely inwards into the brain substance, often carrying with them cells 

 of the pars intermedia for a short distance along their course. The 

 intimate relation of the cells of the pars intermedia to pars nervosa, and 

 their differences in .structure from the cells of the anterior lobe proper, 

 appear to indicate that they are physiologically as well as anatomically 

 connected with the brain. In the cat they are aggregated around the neck 

 and body of the posterior lobe, which are hollow and in connnunication 

 with the third ventricle. In animals which have a solid posterior lobe 

 they are disposed more in relation to the brain substance adjacent to the 

 neck, and in the monkey may spread inwards almost to the floor of the 

 third ventricle. 



In experiments which have been made on the physiological action of 

 extracts of the posterior lobe, the material has usually been taken from the 

 pituitary of the ox, on account of its size. An illustration (fig. 8) is given 

 of the developing pituitary of the ox. In this animal the posterior lobe is 

 a thin, solid, elongated structure. The epithelium of the intermediate part 

 spreads widely over its anterior surface, as seen in the figure, but embraces it 

 laterally as well, and passes for considerable distances in the form of columns 

 of cells into the substance of the body of the lobe. Its epithelium is 

 therefore closely bound up with the nervous substance of the lobe during 

 development, and forms an important element in its composition. The 

 question of the derivation of the active physiological principle of extracts 

 of the posterior lobe has been discussed in a previous paper, and reasons 

 have been given for regarding it as in great part derived from the 

 epithelium of the pars intermedia. It is of interest, therefore, to find that 

 in the development of the pituitary of the ox epithelial cells pass freely 

 into the substance of the posterior lobe. 



The disposition of the cells of the pars intermedia is such as to bring 

 them into close relation with the neural canal in the region of the third 

 ventricle. This of course follows from the history of the mode of 

 development of the pituitary, but the spreading of epithelium over the 

 surface and into the brain itself seems to indicate some further connection 

 between the two. In fishes, e.g. the cod, the epithelium of the anterior 

 lobe appears to have the same intimate connection with the nervous 

 tissue of the posterior lobe which obtains in mammals and birds. The 

 posterior lobe is hollow and has connected with it a large saccus 

 vasculosus lined with folds of columnar epithelium. The saccus vasculosus 

 is said by all who have worked at its development to be derived from the 



