466 MALL. [Vol. V. 



Between the auditory vesicle and the after-brain lies the upper 

 ganglion of the glosso-pharyngeal which receives fibres from a 

 group of cells lying in the floor of the after-brain, and sends a 

 bundle of fibres more dorsally into the same. On the ventral 

 side there is a bundle of nerves which communicates with a sec- 

 ond ganglion, the ganglion petrosum. The ganglion petrosum 

 is in direct continuity with a slight invagination of ectoderm at 

 the dorsal part of the second branchial groove, sends a branch 

 into the third arch, and communicates with the ganglion jugulare 

 of the vagus. 



The vagus is composed of two enormous ganglia, as shown in 

 PI. XXX. The two ganglia are united by a band of cells, and 

 from the ganglion nodosum a branch passes to the aboral and 

 lateral side of the fourth branchial arch. At the central end of 

 the nerve numerous branches pass into the after-brain. The 

 ganglion jugulare receives at once twigs from the accessory 

 nerve, which soon arrange themselves into a bundle to become 

 fully separated before the ganglion nodosum is reached. 



The accessory nerve arises as a row of bundles between the 

 vagus and first cervical, and emerges from the after-brain mid- 

 way between its dorsal and ventral walls. As the twigs ap- 

 proach the first cervical nerve, the origin becomes more ventral, 

 and are continuous with the ventral root of this nerve. Although 

 the rudimentary ganglion, first described by Froriep as the gan- 

 glion of the accessory, has been verified by His for the human 

 embryo, I cannot find any trace of it in this specimen. I have 

 had no difficulty in finding it in dog and cat embryos, and there- 

 fore must conclude that it is wanting in this embryo. This is 

 what we should expect to find from time to time, especially in 

 an organ which is in process of degeneration. 



The hypoglossal nerve arises as a group of fibres parallel to 

 but more ventral than the accessory. The bundles are arranged 

 to correspond with the myotomes of the head, and on the aboral 

 side arise in common with the accessory and the first cervical 

 nerve. 



The Spinal Nerves. 



The spinal nerves are all distinctly marked by a large dorsal 

 ganglion which sends small bundles of fibres into the cord, and 

 a ventral root which arises from the motor cells in the anterior 



