1879.] On the Skull and its Nerves in the Green Turtle, 329 



clefts are supplied by their respective nerves ; and that the Schneiderian 

 folds are gills.* 



These conclusions, if accepted, will considerably simplify our con- 

 ception of the segmentation of the vertebrate head. As there are no 

 nerves or clefts in front of the olfactory segment, the olfactory nerve 

 must be taken as the most anterior nerve, and the nose as the most 

 anterior cleft. The next cleft is that in front of the maxillo-palatine 

 arch, of which a part probably persists in the adult as the lachrymal 

 duct : the segmental nerve corresponding to this cleft is the third, or 

 oculomotor nerve. Next comes the mouth cleft, supplied by the fifth, 

 or trigeminal, nerve ; and then in succession the clefts supplied by the 

 facial, glossopharyngeal, and pneumogastric nerves. This view of the 

 constitution of the vertebrate head is found to accord well with the 

 later researches of Professor Parker on the morphology of the skeletal 

 elements of the head. 



Some at least of the labial cartilages will probably prove, on this 

 view, to be homologues of the extrabranchials, a comparison that has 

 already been made by Professor Parker. t 



If the olfactory organs are visceral clefts, they must originally have 

 communicated with the mouth cavity. Indications of a former con- 

 nexion of this kind are by no means wanting ; thus in salmon embryos 

 the alimentary canal extends forwards, so as to underlie the nasal sacs : 

 as development proceeds, this anterior prolongation of the mouth 

 cavity gradually shrinks ; it persists for a short time as a pair of csecal 

 diverticula, which ultimately disappear altogether. 



In conclusion, it may be noted that the Schneiderian folds afford an 

 instance, on the theory here maintained, of structures originally hypo- 

 blasts in nature becoming, from changed circumstances, epiblastic. 



II. " On the Development of the Skull and its Nerves in the 

 Green Turtle {Chelone midas), with Remarks on the Seg- 

 mentation seen in the Skull of various types." By Pro- 

 fessor W. K. Parker, F.R.S. Received February 3, 1879. 



In the first paper on the development of the skull of the Yertebrata, 

 published in " Phil. Trans.," I figured and described certain modifica- 

 tions of the skull in the embryos of the African ostrich, which have 

 only received their explanation lately, and this has become possible 

 through what I see in the embryos of the green turtle. 



For these embryos I am indebted to two of our Fellows, namely, 



* Cf. Dohrn, "Ursprung der Wirbelthiere," p. 23. 

 f " Proc. Zool. Soc," vol. x, part iv, 1878, p. 212. 



