TISSUES OF THE BODY. 



361 



(jlands also, the first rudiments of which may be observed somewhat 

 earlier than in the preceding case, are likewise solid lateral growths of 

 the undermost cells constituting the rudiments of the embryonic hair 

 follicle, and possess the same flask-like form. The cells in their interior 

 begin very early to undergo that so characteristic fatty infiltration with 

 which we are already acquainted, at the same time increasing in volume. 

 Then by continuous growth they gradually form those vesicular lobules 

 met with in the fully matured structure,?. The mammary glands, again, 

 are developed in a manner precisely similar, from the fourth and fifth 

 month on. Around the several aggregations of cells (fig. 354) an 

 external connective-tissue envelope may be seen, a doubling in of the skin. 

 But it is only at the period of puberty and pregnancy that the organ 

 attains a state of perfect development. 



Fig. 353. Sweat glands of a foetus Fig. 354. The mammary gland from a toler- 



at five months, a, superficial, 6, ably mature embryo, after Longer, a, the 



deeper layer of the epidermis; the middle nodulated portion with shorter excre- 



rudiments of the gland are formed scences; 6, and longer, 

 by the exuberant growth down- 

 wards of the latter. 



As to the germ-producing glands, the ovaries and testicles, as far as 

 concerns their cellular elements, we are still, unfortunately, in the dark 

 in spite of numerous investigations. 



Besides those just mentioned, there are a great number of organs of 

 the same nature, whose development takes place on a precisely similar 

 plan, from the so-called intestinal glandular embryonic plate. Among 

 these may be reckoned the glands of the digestive apparatus and the 

 larger organs connected with the latter, e.g., the liver, pancreas, and lungs. 

 Here, instead of the cells of the corneous layer, we have before us the 

 elements of the glandular leaf arranged over the surface of the tube as 

 intestinal epithelium. The mode of formation of these, however, is but 

 imperfectly known, as, for instance, that of the peptic glands and follicles 

 of the large intestine. The follicles of Licberkuhn appear, on the other 

 hand, to consist, from the very commencement, of hollow duplicatures. 

 The first rudiments of Brunner's glands, however, as well as those of the 

 remaining racemose mucous glands, are formed of solid masses of cells. 

 The salivary glands seem to be formed on an analogous plan of develop- 

 ment, except that a far more extensive proliferation of the cells takes 

 place, producing roundish aggregations of the form of the vesicles of the 

 organ. The pancreas commences also in a hollow duplicating whose 

 clothing of cells gives rise by a similar process to the various lobules and 



