SECRETING STRUCTURES. 425 



6th, 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 development ac- 

 cording 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 whicli do not possess germinal spots in particular 

 situations, but in which these spots are diffused more uni- 

 formly over the whole internal surface of the ducts. The 

 human kidney is a gland of this order.* 



We require renewed observations on the original develop- 

 ment of glands in the embryo. From the information we 

 possess, however, 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 

 protrusion 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 com- 

 municate with the ducts. 



* " 1 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 Janthina 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 laminse within certain glands, have a similar 

 arrangement of germinal spots?" Trans. Roy. Soc. Edin. 1842. 



