No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 95 



the nerve leaves, as it atrophies, on either side of the brain. 

 These cells soon vanish, but Dohrn suggests that they may be 

 the remains of a pristine trochlear ganglion. The position and 

 early disappearance of the cells makes me think them homo- 

 logues of the fragments (sec. 8, IV 2 ) found in Acanthias at the 

 side of the brain, rather than homologous to the cells lying 

 above the superior oblique muscle ; for the latter do not soon 

 disappear, but may be found after the entire trochlear nerve is 

 re-established. They represent probably but part of the original 

 trochlear ganglion, the greater part of which is to be looked for 

 in the anterior portion of the Gasserian ganglion. 



In the reconstructions, showing the development of the eye- 

 muscles, I have indicated the position and general direction of 

 the anterior cranial nerves at the several stages reconstructed. 

 Although this was done with the view to establish more surely 

 the relations of the head cavities, and although many large 

 branches have been omitted, the reconstructions may still be 

 helpful in showing the relations of such nerves as are there out- 

 lined. Fig. 1, PI. IV, shows the position of the trochlear and 

 trigeminal nerves at a stage somewhat earlier than that from 

 which sec. 9, PL VI, is drawn. In Figs. 2, 3, 4, the distal por- 

 tion of the trochlearis (V, an.) is seen as it extends forwards 

 over the anterior prolongation of the mandibular cavity or over 

 the superior oblique eye-muscle. 



Between the stages represented in Figs. 4 and 5, and long 

 after all trace of the proximal part of the primary trochlearis 

 has vanished, a small fibrous nerve (sec. 1 1) grows from the 

 brain at the point where the original trochlearis grew from the 

 neural crest. The nerve can be followed a short distance into 

 the mesoderm, but becoming extremely attenuated, is soon lost. 

 Sec. 1 1 shows that a fibrous commissure, crossing the dorsal 

 wall of the brain, connects the two nerve roots (IV) of opposite 

 sides. Soon after the appearance of this small nerve, which is 

 the root of the permanent trochlearis, cells are proliferated to 

 meet it, from the ganglion cells that lie above the superior 

 oblique muscle. Thus the permanent trochlearis (sec. 7, PI. V) 

 arises from two sources, from the brain and from ganglion cells. 



At least two objections may be raised on the ground of a 

 priori improbability against this account of the development of 

 the trochlearis. In the first place, it seems highly improbable 



