THE EMBRYOLOGY OF THE OPOSSUM 133 



Advances in the nervous system. All cranial nerves 

 (fig. 42, reconstruction) are present in stage 30 except the 

 motor nerves of the eye. The olfactory is represented by 

 only a few fibers from the olfactory pit to the forebrain. 

 The optic is represented in a sense by the optic stalk, but the 

 nerve fibers proper are only beginning to appear. The retina 

 shows the first steps of differentiation of the cellular and 

 fibrous layers. The gasserian ganglion has a strong connec- 

 tion with the hind-brain, and the three branches of the tri- 

 geminal (ophthalmic, maxillary, and mandibular) are well 

 formed. The facial nerve has not yet developed a chorda tym- 

 pani. The dendrites of the eighth nerve from the otocyst are 

 now separable for the first time into two groups, a dorsolateral 

 or superior group, and a ventrolateral or inferior group. The 

 geniculate and acoustic ganglia are fused dorsally, but their 

 ventral thirds are separated, the acoustic being medial to the 

 geniculate. The anlage of the spiral ganglion is recognizable 

 as a less compact bud on the ventrolateral tip of the acoustic 

 complex. The glossopharyngeal nerve has a well-developed 

 petrosal ganglion, but the superior ganglion is indistinct and 

 diffuse. The vagus similarly shows a good nodosal ganglion, 

 but the jugular accessory root ganglia are poorly formed. 

 The spinal accessory is well developed proximally but not 

 separated from the vagus distally. The hypoglossal can be 

 followed all the way to the tongue anlagen. It may arise 

 either lateral or medial to the precardinal vein. If it arises 

 lateral, then during stage 30 it migrates through the pre- 

 cardinal, or the latter grows around it, so that the nerve be- 

 comes medial. It is always medial in stage 31 and later. 

 Froriep's ganglion is present, but I have not seen any con- 

 nection between it and the hypoglossal. 



The first cervical nerve has no ganglion at any time. The 

 fourth, fifth, sixth, seventh, eighth cervicals and the first 

 thoracic form a loose and irregular fusion which is the pri- 

 mordium of the brachial plexus. There is no indication of a 

 lumbosacral plexus as the nerves at that level are just be- 

 ginning to grow out, and the ganglia are just becoming 

 organized from the neural crest. 



