SECRETING STRUCTURES. 419 



cells, explains a difficulty which has hitherto puzzled physio- 

 logists viz. why a secretion should only be poured out on the 

 free surface of a gland-duct or secreting membrane. 



"Why," says Professor Miiller, "does not the mucus 

 collect as readily between the coats of the intestine as exude 

 from the inner surface ? Why does not the bile permeate the 

 walls of the biliary ducts, and escape on the surface of the 

 liver, as readily as it forces its way outwards in the course of 

 the ducts ? Why does the semen collect on the inner surface 

 only of the tubuli seminiferi, and not on their exterior, in 

 their interstices? The elimination of the secreted fluid on 

 one side only of the secreting membrane viz. on the interior 

 of the canals is one of the greatest enigmas in physiology." 

 Mliller proceeds to explain this enigma by certain hypotheses ; 

 but the difficulty disappears, the mystery is removed, when 

 we know that the secretion only exists in the interior of the 

 ripe cells of the free surface of the ducts or membrane, and is 

 poured out or eliminated simply by the bursting and solution 

 of these superficial cells. 



I have hitherto confined my observations to the structure 

 and function of the ultimate secreting element, the primary 

 secreting cell. I now proceed to state the laws which I have 

 observed to regulate the original formation, the development, 

 and the disappearance of the primary organ. This subject 

 necessarily involves the description of the various minute 

 arrangements of glands and other secreting structures. 



If the testicle of Squalus cornubicus (Gmelin) be examined 

 when the animal is in a state of sexual vigour, the following 

 arrangements of structure present themselves : 



The gland consists of a number of lobes separated, and at 

 the same time connected, by a web of filamentous texture, in 

 which ramify the principal bloodvessels. 



The lobes, when freed from this tunic, present on their 

 surface a number of vesicles. When the gland is dissected 



