THE ORGANS OF THE OUTER GERM-LAYER. 



495 



was early ascertained by the zoologist RATHKE in the case of Coluber. 

 Hecently KRAUSE has still farther elucidated the interesting pro- 

 cesses by the construction of wax models of the conditions in 

 mammalian embryos. 



As is to be seen from the various sections (figs. 277, 278), but still 

 better from the model (fig. 276) produced by reconstruction, the 

 semicircular canals are developed by the protrusion of several evagina- 

 tions of the wall of the sac, which have the form of thin pockets or 

 discs (hb, vb) with a 

 semicircular outline. 

 The marginal part of 

 each such evagina- 

 tion now becomes 

 con siderably en- 

 larged, whereas the 

 remaining portions 

 of the two epithelial 

 layers come into close 

 contact and begin to 

 fuse. As the result 

 of this simple process 

 the enlargement at 

 the margin and the 

 fusion of the walls 

 which takes place in 

 the middle there is 

 formed a semicircular 

 canal, which commu- 

 nicates at two places 

 with the original 

 cavity of the vesicle. 

 A.t one of its open- 

 ings the canal is early enlarged into an ampulla (fig. 276 am 

 -and am'). The middle part, in which the fusion has taken place, 

 soon disappears, the epithelial membrane being broken through by a 

 growth of the connective tissue (fig. 276 o). 



There exists an interesting difference between the development of 



the horizontal and the two vertical canals, which was discovered by 



KRAUSE. Whereas the horizontal canal is established as a small 



pocket by itself (fig. 276 /ib), the two vertical canals arise together 



from a single large pocket-like fundament (fig. 276 am (vb), *, vb'). 



Fig. 278. Cross section through half of the head of a foetal 

 Sheep 2 cm. long, in the region of the labyrinth, after 

 BOETTCHER. Magnified 30 diameters. 



rl, Recessus labyrinth! ; vb, lib, vertical and horizontal semi- 

 circular canals; U, utriculus ; /, inward-projecting fold, 

 by which the labyrinth-sac is divided into utriculus and 

 sacculus ; do, ductus cochlearis ; gc, ganglion cochleare. 



