GLANDS 



59 



I. Epithelial glands, with persistent ducts, producing external secretions. 



1. Unicellular glands. 



2. Simple glands. 



a. Ectodermal, e.g., sweat and sebaceous glands. 



b. Mesodermal, e.g., uterine glands. 



c. Entodermal, e.g., gastric and intestinal glands. 



3. Compound glands. 



a. Ectodermal, e.g., mammary and lachrymal glands. 



b. Mesodermal, e.g., epididymis and kidney. 



c. Entodermal, e.g., pancreas and liver. 



II. Epithelial glands, with obliterated ducts, producing internal secretions. 



a. Ectodermal, anterior lobe of the hypophysis (the duct of the 



posterior lobe is partially obliterated). 



b. Entodermal, thyreoid gland. 



III. Epithelioid glands, never having duct or lumen, producing internal 

 secretions. . 



a. Ectodermal (through their relation to the sympathetic nerves), 



chromaffin bodies; and medulla of the suprarenal gland. 



b. Mesodermal, cortex of suprarenal gland; interstitial cells of 



the testis; corpus luteum. 



c. Entodermal, islands of the pancreas; epithelioid bodies in 



relation with the thyreoid gland; thymus (?) 



IV. Cytogenic glands, producing cells. 



a. Mesodermal, epithelial the ovary and testis. 



b. Mesodermal, mesenchymal the lymph glands, haemolymph 



glands, spleen, red bone marrow, and many smaller lymphoid 

 structures. 



THE MESENCHYMAL TISSUES. 



Mesenchyma (/wos middle, trxyf^, an infusion) is a term introduced 

 by O. Hertwig, in 1883, for the tissue produced by cells which have 

 wandered out from the epithelial germ layers into the spaces between them. 

 It is found only in young embryos. In the adult it is represented by a 

 large group of derivatives, including connective tissue, adipose tissue, 

 cartilage, bone, smooth muscle fibers, tendons, fasciae, and various 

 special forms of cells. Mesenchyma arises chiefly from different parts 

 of the mesoderm, as already described (p. 42), but in the head of the 

 chick embryo a portion of it comes from the ectoderm, and in the wall of 

 the intestinal tube, according to Hertwig, the entoderm contributes to 

 its formation. Together with the blood islands it constitutes the entire 

 non-epithelial tissue of the embryo in early stages. It consists of a net- 



