1879.] 



Skull and its Nerves in the Green Turtle. 



337 



the outgrowing limbs. I suspect, indeed, that they are their true 

 serial homologues. 



In this latter case, the splitting of the mesoblasts into several strata 

 in the throat, on the one hand, and in the thoracic and pelvic regions 

 of the body, on the other, are strictly comparable morphological 

 changes. 



I strongly suspect that if we could bridge over the gulf between 

 the lancelet and the lamprey, we should find in these " connecting 

 links " that the head was not a mere repetition of the body. 



Somatomes we should find ; but the intercalary skeletal parts would 

 be found, I believe, to run into each other from the first, and the hasi- 

 neural cartilages of the head might be seen, in time, before the 

 distinct neural arches of the spine. 



I must now recapitulate a little. 



After carefully considering the views and studying the researches 

 of Huxley, Gregenbaur, Balfour, and Milnes Marshall, I am satisfied 

 that, in spite of the doubling up of the basis cranii, at the time of its 

 greatest flexure, there are rudiments of three pre-oral arches, related to 

 two pre-oral clefts, namely, the lacrymal and the nasal. 



That the mouth is caused by the blending together of a right 

 and left cleft is the view held (I find) by Dr. Allen Thomson ; this 

 view, also, is held by Dr. Dohrn. (See " Balfour's Elasmobranchs," 

 p. 15.) 



The horseshoe fold of the mid-brain, the formation of the large 

 hollow bed for the eyeball, and the special function to which the true 

 segmental motor oculi nerve is confined, are all correlates of the 

 special development of those wild branches of the trigeminal and 

 facial nerves, namely, the ophthalmic and Vidian. 



This is tantamount to saying that the hypertrophy of the first and 

 second vesicles of the brain, and the large size of the optic vesicles 

 which are outgrowths from the first of these, with all the enfoldings 

 and complications necessary to complete the eyeball have, together, 

 dominated all the surrounding parts, causing them to do many strange 

 things, so to speak, vicariously. 



Milnes Marshall finds that the olfactory nerves are solid until the 

 seventh day of incubation in the chick; in embryos of the green 

 turtle of the size of a horse-bean, I find the nerves still solid. 



When the embryos are two or three times that size, these nerves 

 each acquire a large cavity proximally, from the fore-wall of which 

 the branches seem to spring. 



The foremost of these branches spring from the top of the vesicle : 

 they arose at first from the top of the fore-brain. 



Both in the chick and embryo of the turtle, the fourth nerve, as 

 soon as it can be found, runs a course so directly athwart the first 

 branch of the fifth as to suggest its non-segmental nature. 



2 b 2 



