ELEMENTS OF STRUCTURE. 



85 



In our opinion, the latter glass-like subepithelial strata have nothing to 

 do with the cells of this tissue, they are more probably modified limiting 

 layers of the connective tissue of cornea and mucosa. 



Fig. 77. Glands from the large intestine 

 of the rabbit; one follicle with cells 

 and four glands, of .which only the 

 Membrana propria has remained com- 

 pletely stripped of cells. 



Fig. 78. Follicle from the large intestine of the 

 guinea-pig. Gland at a, with Membrana pro~ 

 pria partially visible ; at 6, the contents have 

 escaped through a slit in the latter membrane. 



As we have just remarked, there occur, enveloping certain groups of 

 cells, homogeneous layers, constituting, especially among the glandular 

 structures, what is known as the 

 Membrana propria, i.e., a trans- 

 parent tunic investing the gland 

 und determining its shape, us 

 well as that of its several parts, 

 and on this account of much im- 

 portance. Of these membranes 

 are formed the vast multitude 

 of follicular glands, having the 

 shape of long narrow pouches 

 (figs. 77, 78 ), whilst in the no 

 less widely distributed group of 

 racemose glands the latter are 

 replaced by numbers of flask- 

 shaped saccules packed closely 

 together (fig. 79). 



But also around aggregations 

 of embryonic cells, destined 

 later on to become definite 

 structures, similar transparent 

 envelopes are to be found, as, 

 for instance, around the rudi- 

 mentary human hair, as pointed out by Koelliker (fig. 80). 



Such homogenous membranes have been regarded by some as produced 

 by the solidification of a secretion from the cells themselves, a theory 

 which is not tit all weakened by the fact of the separation of the trans- 

 parent envelope from the cells by which it is formed, nor that it outlasts 

 the elements from which it has had its origin. It is, however, difficult to 

 explain why in an aggregation of cells identically the same, only those 

 situated externally should possess the power of generating such a secre- 

 tion. 



Fig. 79. A human racemose gland (Brawler's) with 

 saccules of the Membrana propria. 



