182 Herring 



explanation. It is possible that the epithelial cells secrete some substance 

 which is necessary for the brain. Andriezen's view that the secretion of 

 the cells of the anterior lobe of the pituitary is carried by lymphatics and 

 blood-vessels to the brain is unlikely, owing to the vascular arrangements in 

 the lobe ; but it is probable that something of the kind occurs with the secre- 

 tion of the cells of the pars intermedia. B. Haller regards the anterior lobe 

 as a tubular gland which provides a secretion for the membranes of the 

 brain and spinal cord. It is remarkable that there should be so large a 

 cleft in the pituitaries of the dog and cat, unless it has some function. An 

 external opening of the cleft is frequently seen, but it may quite well be 

 an artiticial one, and in carefully prepared specimens I have been unable 

 to tind it. The anterior lobe is not a tubular gland, and the only cells that 

 can pour a secretion into the cleft are those of the pars intermedia. On 

 the other hand, the cleft is not always well developed even in the cat's 

 pituitary, and may be almost entirely closed by fusion of the anterior lobe 

 with the cells of the epithelial covering of the lobe. In the pituitary of 

 the monkey there may be little remnant of the cleft, and certainly no 

 opening from it into the subdural space. It is rare to find any histological 

 evidence of a secretion into the cleft, and where colloid has been present it 

 has been enclosed in a thin- walled cyst and not lying free in the cleft. In 

 the rabbit's pituitary it is not uncommon to find the cleft filled with red 

 bone-marrow and fat cells. Lymph spaces in the dura mater below the 

 pituitary body are frequently seen, but there is no evidence that they are 

 specially connected with it : they probably belong to a system of lymphatics 

 present at the base of the brain. 



Conclusions and Summary, 



Development of the pituitary body begins very early in embryonic life. 

 In mammals the epithelial portion is derived entirely from the ectodermic 

 wall of the buccal invagination known as Rathke's pouch. Its origin is 

 single and mesial. The epithelium is ditierentiated at an early stage into 

 two parts, which show difierences in arrangement, structure, and vascularity. 

 One of these, which has been termed the pars intermedia, is closely adherent 

 to the wall of the cerebral vesicle from its earliest appearance, and remains 

 attached to it throughout life. It forms a layer of cells of varying thick- 

 ness over body and neck of the posterior lobe and adjacent parts of the 

 brain, and tends to arrange itself in positions where it can approach as near 

 as possible to the cerebro-spinal canal. The cells of the pars intermedia 

 are further characterised by the absence of deeph^ staining granules from 

 their protoplasm, by their tendency to form a colloid substance in the adult 

 organ, and by their compai-atively poor supply of blood-vessels. Its relation 

 to the nervous part of the pituitary and to the adjacent wall of the brain 

 tends to become even more intimate as development proceeds, by the 

 ingrowth of its cells into these structures. 



