ORIGIN OF THE GLANDS. 



265 



packed between a very delicate external envelope turned to- 

 wards the blood-vessels, and an internal epithelial investment. 

 The cellular structure of the parietes of the ventricular glands 

 is, however, very apparent in young birds (fig. 186, B). In 

 other glands, moreover, we recognize the cellular structure 

 with different degrees of distinctness in the tubuli uriniferi, 

 for example, where the cells have nuclei, but are far from 

 being so compact, and are not nearly so readily isolated as in 

 the liver (fig. 261). It is difficult to say in how far this cel- 

 lular structure, which may be followed to the very ends of 

 the canaliculi, belongs to the innermost layer of the glandular 

 paries, or is connected with the epithelial investment, ap- 

 pertaining to the trunk and larger branches of the excretory 

 duct of every gland. Apparently, however, there are always 

 several layers of flattened cells placed one upon another, over 

 which a structureless membrane is drawn externally, and this 

 is the part that is surrounded immediately by the vascular 

 reticulation. Certain it is, that wherever we find secreting 

 follicles, they consist of a number of more or less distinctly 

 cellular or fibrous layers, which lie as the proper substance 

 of the gland betwixt the external net-work of blood-vessels 

 and the inner wall whence the secreted matter distils away. 



ORIGIN OF THE GLANDS. 



[ 427. Thegreater number of the secreting glands arise from 



Fig. 2G9. Rudiments of the 

 liver formed by evolution from the 

 tractus intestinalis in the embryo 

 of the fowl of the fourth day. 

 After Miiller De Gland, &c. 



Fig. 270. Liver and pancreas 

 of an embryo of the fowl at the 

 end of the fourth day, magnified 

 twelve times linear, a, the liver; 

 b, the pancreas; c, the stomach; 

 d, d, the lungs. 



