»•] 



THE CEANIAL NERVES. 



137 



Schenk (Wien. Sitz. Bericht. 1868) describes all the cells which invest 

 the hypoblast of the digestive tract, as primarily derived from the proto- 

 vertebra?, with the exception of the peritoneal epithelium, which alone, he con- 

 siders, is the representative of the original mesoblast of the splanchnopleure. 

 According to this view, the muscles of the walls of the alimentary canal, and the 

 'Irypaxial' muscles, are derived from the original proto vertebras, quite as much 

 as those muscles which spring out of the muscle-plate. In the absence of any 

 satisfactory means of distinguishing the cells of the intermediate cell-mass 

 from those of the protovertebraa, this view must be considered as at least very 

 doubtful. 



Of! 



— ■ ' . 



In the mesoblast, which lies by the side of the 

 hind brain, and which though not divided into protover- 

 tebrae is the prolongation forwards of the same column of 

 mesoblast out of which in the trunk the protovertebra? are 

 formed, there appear on either side in the course of the third 

 day a series of four small opaque masses, somewhat pearshaped 

 with the stalk directed away from the middle line. These 

 are the rudiments of four cranial nerves, of which two lie in 

 front of and two behind the auditory vesicle. 



The most anterior of these is the rudiment of the fifth 

 nerve (Figs. 25, V. 45, V). Its narrowed outer portion or 



Fig. 45. 



-vm 



Head of an Embryo Chick of the Third Day (seventy-five hours) viewed 



SIDEWAYS AS A TRANSPARENT OBJECT (from Huxley). 



la. cerebral hemispheres. lb. vesicle of the third ventricle. II. mid-brain. 



III. hind-brain, g. nasal pit. a. optic vesicle. 6. otic vesicle, d. infundi- 



bulum. e. pineal body. h. notochord. V. fifth nerve. VII. seventh nerve. 



VIII. united glossopharyngeal and pneumogastric nerves. 1, 2, 3, 4, 5 the 



five visceral folds. 

 The stage here represented is a little later than that shewn in Eig. 25, with 

 which it should be compared. 



