THE DUCTLESS GLANDS 305 



etc.), or they have no lumen at all (as in the adrenal, etc.)- We shall 

 take up the descriptions of these tissues in the order in which they are 

 mentioned above. 



Several important secreting tissues are associated, in the vertebrates, 

 with the walls of the brain. In earliest embryonic life the lower brain 

 wall is invaginated to form a longer or shorter ventral depression, the 

 infundibulum, whose lower end becomes cut off to form the hypophysis. 

 The posterior wall of this infundibular invagination remains thin, 

 and becomes of a complicated sac-shaped form, in some animals 

 with the secretory epithelial layer lining a lumen that remains in connec- 

 tion with the brain cavity, while the proximal surface of this epithelium 

 is brought into contact with an abundant blood supply in sinusoids. 

 This structure is called the infundibular gland or, less properly, the "sac- 

 cus vasculosus." 



An invagination of the roof of the oral cavity of the embryo arises, 

 and reaches up until its fundus comes into a close relation with the 

 hypophysis. The fundus of this invagination becomes cut off and de- 

 velops into a mass of cellular cords, being known as the glandular lobe 

 of the hypophysis. 



The neural lobe of the hypophysis generally consists of a fibrous 

 medulla and a cortical region of cells which may have some glandular 

 function, although none has been actually determined. 



We shall first pay particular attention to the infundibular gland 

 as found in the lower vertebrates and then to the glandular lobe of the 

 hypophysis as seen in one of the higher forms. 



The infundibular gland of a flounder, Pseudopleuronectes Americanus, 

 begins in the young embryo as a sac-like invagination of the posterior 

 wall of the infundibulum. The walls of this sac, therefore, consist of 

 an epithelium which was previously invaginated from the ectoderm 

 to form the brain and would otherwise have become nervous in function. 

 This sac-like invagination becomes folded and bent by many septa that 

 arise from its walls. With each fold there comes a blood-vessel loop 

 which enlarges to become a thin-walled sinusoid. The whole number 

 of vessels soon form a plexus that carries a large quantity of blood and 

 gives the structure its characteristic red color during life. 



Figure 275 shows parts of two rows of the secretory cells and parts of 

 the blood spaces upon which they lie. These latter are filled with the 

 typical nucleated corpuscles of the fishes. The cells are large and heavy 

 with a dense cytoplasm that shows no vacuoles or granules, or other 

 evidences of activity. The nuclei are round, and placed slightly toward 

 the proximal end of the cell. The nucleolus lies almost exactly in the 

 center of the nucleus, and the chromatin is arranged in strands which 

 radiate with some regularity from the nucleolus to the nuclear mem- 



