202 bulletin: museum of comparative zoology. 



One of these main branches is the latter ramus, the one innervating the 

 dorsal rectus muscle ; the other is the ventral ramus, wliich extends to 

 the ventral oblique muscle. The ciliary ganglion should, then, be con- 

 sidered as sessile on the ventral division of the nerve. 



At seven days I have seen no evidence of the ciliary nerve. At 

 eighteen days it is present as a comparatively large bundle of fibrils, 

 which runs from the ciliary ganglion to the eyeball. The fibrils pene- 

 trate the sclerotic coat caudad of the entrance of the optic nerve, and 

 continue their course between the sclerotic and choroid tunics. 



In the seven-days' stage the difference in size between the ganglion, 

 cells of the ciliary and Gasseriaa ganglia is striking. 



In the ciliary ganglion of an eighteen-days' embryo, small cells with 

 crescentic nuclei can be seen arranged about the ganglion cells. Their 

 processes are continuous, forming a complete envelope, the nucleated 

 capsule of the ganglion cell. Eetzius ('81) figures these nucleated cap- 

 sules around the bipolar ciliary-ganglion cells of the adult fowl. 



4. Abducent Nerve. The sixth nerve had reached in Stage V its 

 adult condition, as far as its relations to the posterior rectus muscle and 

 surrounding structures were concerned, except that its branch to the 

 muscles of the nictitating membrane had not appeared. I have not 

 observed the formation of this branch. It could not be found at the 

 end of seven days' incubation. 



5. Hye Muscles. As has been said in the description of the ocu- 

 lomotor nerve, at some period between Stage V (about one hundred 

 nineteen hours) and one hundred sixty-eight hours, the common funda- 

 ment of the ventral and anterior rectus m\iscles becomes divided into 

 two parts, from which develop the separate muscles of the adult. 



6. Trochlear Nerve. In sections of seven-days' embryos, the fourth 

 nerve can be traced from its dorsal superficial origin between the mid- 

 brain and the hind-brain to its termination in the dorsal oblique muscle. 

 Before reaching the muscle the nerve spreads out into a loose brush of 

 fine fibrils, which, even in vom Eath preparations, are not easily traceable 

 through the mesenchyme. 



Discussion of Results. 



Migration of Medullary Cells. Frequent allusions to the migration of 

 cells from the embryonic neural tube are to be found in neurological 

 literature since the time of Balfour. That pioneer among the investi- 

 gators of the histogenesis of nerves believed he found in elasmobranchs 

 clear evidences of the escape of nerve-forming cells from the spinal cord 



