32 SECRETING STRUCTURES. 



ever, it appears that the process is identical in its nature with the 

 growth of a gland during its state of functional activity. 



The blastema, which announces the approaching formation 

 of a gland in the embryo, in some instances precedes, and is in 

 other instances contemporaneous with, the conical blind pro- 

 trusion of the membrane upon the surface of which the future 

 gland is to pour its secretion. 



In certain instances it has been observed that the smaller 

 branches of the duct are not formed by continued protrusion of 

 the original blind sac, but are hollowed out independently in the 

 substance of the blastema, and subsequently communicate with 

 the ducts. 



It appears to be highly probable, therefore, that a gland is ori- 

 ginally a mass of nucleated cells, the progeny of one or more 

 parent cells ; that the membrane in connexion with the embryo 

 gland may or may not, according to the case, send a portion of 

 the membrane, in the form of a hollow cone, into the mass ; but 

 whether this happens or not, the extremities of the ducts are 

 formed as closed vesicles, and then nucleated cells are formed 

 within them, and are the parents of the epithelium cells of the 

 perfect organ. 



Dr. Allen Thomson has ascertained that the follicles of the sto- 

 mach and large intestine are originally closed vesicles. This 

 would appear to shew that a nucleated cell is the original form of 

 a follicle, and the source of the germinal spot which plays so im- 

 portant a part in its future actions. 



The ducts of glands are therefore inter-cellular passages. This 

 is an important consideration, inasmuch as it ranges them in the 

 same category with the inter-cellular passages and secreting re- 

 ceptacles of vegetables.* 



Since the publication of my paper on the secreting structures, 

 in the Transactions of the Royal Society of Edinburgh in 1842, 

 l^have satisfied myself that 1 was in error, in attributing to the 

 cell wall the important function of separating and preparing the 

 secretion contained in the cell cavity. The nucleus is the part 



* Honle, in his General Anatomy, has made a similar statement. 



