164 bulletin: museum oe comparative zoology. 



first makes its appearance as a number of crowded psile cells in the mar- 

 ginal veil of the mid-brain. The plasma of these cells emerges from 

 the ventral side of the neural tube as fine processes, which unite to form 

 an irregular network. The meshes of this network are extended by the 

 fusion of the processes of large cells, the nuclei of which lie in the 

 plasma mass. The network stretches out through the mesenchyme, and 

 gives rise, in the vicinity of the front end of the chorda dorsalis, to the 

 small trunk of the oculomotor nerve. Dohrn's view of the structure of 

 the growing nerve is in general accordance with the opinions of Balfour, 

 Marshall, Beard, von Kupfifer, and others, who support the "chain 

 theory" of nerve formation. To prove that the medullary tube is the 

 source of these cells whose processes make up the nerve trunk, Dohm 

 devotes much space and many figures. He shows that cells may be ob- 

 served in the root of the oculomotor nerve, half in and half out of the 

 medullary tube. Although he is obliged to admit that with present 

 staining methods it is not possible, in early embryonic stages, to dis- 

 tinguish emigrating medullary cells from the surrounding mesodermal 

 cells, he calls attention to the fact that the nuclei of the nervous net- 

 work are larger than the nuclei of nearly all the neighboring mesoder- 

 mal cells. Numbers of rounded and oval nuclei are to be seen in the 

 course of the oculomotor before this grows down and connects with the 

 mesocephalic ganglion, the long axes of these niiclei being perpendicular 

 to those of the nuclei of the ganglion. The nuclei lying along the third 

 nerve cannot, therefore, be considered derivatives of the mesocephalic 

 ganglion. 



In older embryos there occur, in the course of the third nerve, groups 

 of differentiating ganglion cells, corresponding in position to the ganglia 

 found along the oculomotor in the fully grown animal ; and the develop- 

 ment of these cells can be followed with certainty until the adult condi- 

 tion is reached. The ganglion cells arising in this way Dohm believes 

 to have been originally migrant medullary cells. 



After considering the definitions which have been given to cerebro- 

 spinal and sympathetic ganglia, Dohm reaches tlie conclusion that in 

 the diffuse ciliary ganglion of selachians we have a ganglion which, 

 because of its unique origin from emigrant medullary cells, belongs 

 neither to the cerebro-spinal nor to the sympathetic systems. 



In its histogenesis, the abducent nerve was found to resemble closely 

 the oculomotor. Cells were discovered, wandering out into its roots 

 from the ventral wall of the hind-brain, but in the case of this nerve 

 none of these become ganglion cells in the adult. 



