EMBBYOLOGY. 



Fig. 39. Diagram of the formation if glands. 



1, Simple tubular gland ; 2, branched tubular gland : 3, 



branched tubular gland with anastomosing branches ; 



4 and 5, simple alveolar glands ; a, duct ; db, vesicular 



enlargement ; 6, blanching alveolar gland. 



narrow and tube-like and serves as its duct (a). More complicated 

 forms of glands arise, when the same processes to which the simple 

 glandular sac owes its origin are repeated on the wall of the sac 



when on a small 



12 3 4 5 6 



tract or it a more 

 vigorous growth 

 again takes place, 

 and a part begins 

 to grow out from 

 the main tube as a 

 lateral branch (fig. 

 39 2 and 6 ). By 

 numerous repetitions 

 of such evaginations, 

 the originally simple 

 tubular gland may 

 acquire the form of 

 a much - branched 

 tree, upon which we 

 distinguish the part 



formed first as trunk, and the parts which have arisen by outgrowths 



from it as chief branches and branchlets of first, second, third, and 



fourth order, according to their ages and correlated sizes. According 



as the lateral outgrowths remain tubular or become enlarged at their 



tips, there arise either the compound tubular 



glands (fig. 39 2 ) (kidney, testjs, liver), or the 



compound alveolar glands (fig. 39 6 ) (sebaceous 



glands of the skin, lungs, etc.). 



Again, the invaginating part of an originally 



flat membrane assumes other forms in the pro- 



duction of sense organs and the central nervous 



system. For example, the part of the organ of 



hearing which bears the nerve terminations 



the membranous labyrinth is developed out of 



a small tract of the surface of the bcdy, which 



becomes depressed into a small pit (fig. 40) in 



consequence of its acquiring an extraordinary 



vigor in growth. The edges of the auditory 



pit then grow toward one another, so that this is gradually con- 



verted into a little sac, which still opens out at the surface of the 



body by means of a narrow orifice only (fig. 40 a). Finally, the 



Fig. .-Diagram of the 

 formation of the audi- 

 tory vesicle. 



a, Auditory pit ; b, audi- 

 tory vesicle, which has 

 arisen by a process of 

 constriction, and still 

 remains connected with 

 the outer germ-layer by 

 means of a solid stalk 

 of epithelium. 



