THE ORGANS OF THE OUTER GERM-LAYER. 457 



the fundaments of branchial sensory organs, which no longer attain 

 to complete development. Also FRORIEP, on the strength of his own 

 observations, holds as admissible the interpretation that at those 

 places where fusion occurs formative material passes out of the 

 epidermis into deeper parts to share in the formation of nervous 

 tracts. BEARD expresses himself still more precisely to the effect 

 that the sensory nervous elements of the whole peripheral nervous 

 system arise as differentiations from the outer germ-layer, independ- 

 ently of the central nervous system. 



The accounts here given concerning a connection, in early stages of develop- 

 ment, of certain nerve-trunks with the outer germ-layer, appear to me to afford 

 an indication in favor of the hypothesis expressed by my brother and me, 

 that the sensory nerves of the Vertebrates may have originally been formed 

 out of a sub-epithelial nervous plexus, such as still exists in the epidermis of 

 many Invertebrates. 



(3) The investigations of the last few years, which have been 

 carried out especially by BALFOUR, MARSHALL, KOLLIKER, WIJHE, 

 FRORIEP, RABL, and KASTSCHENKO, have produced important results 

 concerning the development of the cranial nerves, their relations to 

 the head-segments and their value as compared with spinal nerves. 

 On the brain, as well as on the spinal cord, there arise roots, some 

 of which are dorsal, some ventral. Even at the time when the 

 brain-plate is not yet fully closed into a tube (fig. 261), there is 

 formed on either side, at the place of its bending over into the 

 primitive epidermis, a neural ridge (vg), which begins rather far 

 forward and may be traced on serial sections uninterruptedly in a 

 posterior direction, where it is continuous with the neural ridge 

 of the spinal cord. When, somewhat later, the closure and the 

 detachment of the brain-vesicles from the primitive epidermis has 

 taken place, the ridge lies on the roof of the vesicles and is fused 

 with them in the median plane. The most of the cranial nerves 

 namely, the trigeminus with the Gasserian ganglion, the acusticus 

 and facialis with the ganglion acusticum and probably also the 

 ganglion geniculi, and the glossopharyngeus and vagus with the 

 related ganglion jugulare and g. nodosum are differentiated out of 

 this fundament in the same manner as the dorsal roots of the 

 spinal nei-ves. The nerves, which emerge dorsally, afterwards shift 

 their origin downward along the lateral walls of the brain -vesicles 

 toward the base of the latter. 



All the remaining unenumerated cranial nerves oculomotorius, 

 trochlearis, abducens, hypoglossus, and accessorius are developed 



