306 



HISTOLOGY 



M.' 



brane. The distal end of such a secreting cell ends in a knob-like 

 process directed into the gland lumen (which is part of cavity openly 



connected with the 

 brain lumen) and 

 a number of pecul- 

 iar, heavy gran- 

 ules, some of which 

 seem hollow, are 

 collected around 

 this knob like a 

 sort of cap. 



A rather re- 

 markable feature is 

 the number of 

 slightly smaller nu- 

 clei that lie in this 

 epithelium near the 



FIG. 275. Part of a section through the infundibular gland of a distal ends of the 

 flounder, Pseudopleuronectes Americanus. lu., lumen; bl., blood ppllc Thev RDDPar 

 vessels; sec.ep., secreting epithelium. X 750. * ^^ 



to be in the secre- 

 tory cells or between them. Well-preserved preparations show that 

 they belong to smaller cells which lie between the ends of the secre- 

 tory cells and seem to be degenerating in the mature gland. No evi- 

 dences of any renewal process' can be seen that would indicate such 

 a casting off of cells, however. 



This gland evidently secretes some substance into the brain-cavity 

 fluids. It is not the only point, however, at which the brain-wall cells 

 have given up their nervous functions to become gland cells. At other 

 points in the brains of all vertebrates are found places where the wall 

 has been evaginated into long, branching, and anastomosing tubes of 

 simple epithelium, and blood vessels have followed these tubes in and 

 occupy their centers. Thus again the blood is brought into relation 

 with the brain-cavity fluids with only a simple epithelium and the 

 thin blood-vessel endothelium lying between them. It is considered 

 in this case that the epithelial cells are active in removing substances 

 from the brain-cavity fluids. Such a structure is known as a choroid 

 plexus. 



The glandular lobe of the cat's hypophysis will next claim our atten- 

 tion. As was said above, this is an epithelial invagination of the mouth 

 cavity which has become subsequently cut off, and superficially attached 

 to the neural lobe of the hypophysis. It undergoes a complex develop- 

 ment in many animals, and comes to be made up of lobules like the 

 thyroid gland (see below). In the cat, however, the lumen almost dis- 



