SECRETING STKUCTt'RKS. 31 



which take on the action of, and become primary secreting cells, 

 as they advance along the follicle. The action of other follicles 

 is periodical. 



5th, The wall, or germinal menbrane of the follicle, is also in 

 a state of progressive growth, acquiring additions to its length 

 at its blind extremity, and becoming absorbed at its attached 

 extremity. My brother, in a paper on the Developement and 

 Metamorphoses of GaUgus, read in the Wernerian Society, April 

 1842, has stated that the wall of the elongated and convoluted 

 follicle, which constitutes the ovary in that genus, grows from its 

 blind to its free extremity, at the same rate as the eggs advance 

 in developement and position. A progressive growth of this kind 

 would account for the steady advance of its attached contents, 

 and would also place the wall of the follicle in the same cate- 

 gory with the primary vesicle, germinal membrane, or wall of the 

 acinus in the vesicular glands. 



6^, The primary secreting cells of the follicle are not always 

 isolated. They are sometimes arranged in groups, and when 

 they are so, each group is enclosed within its parent cell, the 

 group of cells advancing in developement according to its 

 position in the follicle, but never exceeding a particular size in 

 each follicle. 



In my original memoir, I stated my opinion, that there is an 

 order of glands, namely, those with very much elongated ducts, 

 which do not possess germinal spots in particular situations, but 

 in which these spots are diffused more uniformly over the whole 

 internal surface of the ducts. The human kidney is a gland 

 of this order.* 



We require renewed observations on the original development 

 of glands in the embryo. From the information we possess, how- 



* " I am the more inclined to believe this, from what I have observed in certain secreting 

 membranes. Thus the membranes which secrete the purple in Aplysia and Jantlmui are not 

 covered with a continuous layer of purple secreting cells, but over the whole surface, and at 

 regular distances, there are spots, consisting of transparent, colourless nucleated cells, around 

 which the neighbouring cells become coloured. Are these transparent cells the germinal 

 spots of these secreting membranes ? And may not the walls of the elongated tubes, and 

 the surfaces of the laminae within certain glands, have a similar arrangement of germinal 

 spots ?" Trans. 7?o//. >Sw., AV/.v. 184:?, 



