82 EMBRYOLOGY. 



branchings. The formation of the limbs is also referable to such 

 a process of external budding. 



When the growth of the membrane takes place along a line, 

 the free edges form ridges or folds directed outward, such as 

 the valves of KERKRING or the gill-plates on the gill-arches of 

 Fishes, 



From the examples cited it is clearly to be seen how the greatest 

 variety of forms may be attained by the simple means of invagina- 

 tion and evagination alone. At the same time, the forms may be 

 modified by two processes of subordinate importance, by separations 

 and by fusions which affect the cell-layers. Vesicular and sac-like 

 cavities acquire openings by the thinning out of the wall at a place 

 where the vesicle or sac lies near the surface of the body, until there 

 is a breaking through of the separating partition. Thus in the 

 originally closed intestinal tube of "Vertebrates there are formed the 

 mouth-opening and the anal opening, as well as the gill-clefts in 

 the neck-region. 



The opposite process fusion is still more frequently to be 

 observed. It allows of a greater number of variations. We have 

 already seen how the edges of an invagination may come in 

 contact and fuse, as in the development of the auditory vesicle, 

 the intestinal canal, and the neural tube. But concrescence may 

 also take place over a greater extent of surface, when the facing sur- 

 faces of an invaginated membrane come more or less completely into 

 contact, and so unite with each other as to form a single cell-mem- 

 brane. Such a result ensues, for example, in the closure of the 

 embryonic gill-clefts, in the formation of the three semicircular 

 canals of the membranous labyrinth of the ear, or, as a pathological 

 process, in the concrescence of the surfaces of contact of serous 

 cavities. Moreover fusions may take place between sacs which come 

 in contact with their blind ends, as very often occurs in the com- 

 pound tubular glands (fig. 39 3 ). Of the numerous lateral branches 

 which sprout out from the tubule of a gland, some come in contact 

 at their ends with neighboring branches, fuse with them, and 

 establish an open communication with them by the giving way 

 of the cells at the place of contact. It is by this means that 

 branched forms of tubular glands pass into the net-like forms to 

 which the test is and the liver of Man belong. 



In addition to the formation of folds in epithelial layers, which 

 under a great variety of modifications determine in general the 

 organisation of the animal body, there were mentioned, as a second 



