OEIGIN OF THE GLANDS. 



260 



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. 



OF THE GLATO)S. 



[ 427. The greater number of the secreting glands arise from 



Fig. 269. Rudiments of the 

 liver formed by evolution from the 

 tractus intestinalis in the embryo 

 of the fowl of the fourth day. 

 After Mailer 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. 



