LATER DEVELOPMENT OP THE FROG. 647 



Some Pacts in the Later Development of the 

 Frog, Rana temporaria. 



Part I. — The Segments of the Occipital Region of the Skull. 



By 

 Ag^iics 1. M. Elliot, B.Sc, 



Associate of Newnliam College, Cambridge. 



With Plates 39 and 40. 



The question of the segmentation of the vertebrate head is 

 one which for a long time has attracted the attention of many 

 investigators. With regard to the segmentation of the 

 anterior and middle parts of the head the facts are conflict- 

 ing, and opinions differ; but, with regard to the post-otic 

 portion, there is a converging mass of evidence, drawn from 

 the neural, muscular, and skeletal tissues, that a varying 

 number of segments serially homologous with the trunk 

 segments have been drawn into the head. Besides this there 

 is also evidence that in this region, in some groups at all 

 events, a certain number of segments have altogether dropped 

 out, and are not now represented by any rudiments, even in 

 the embryo. 



In the Cyclostoma the cranium terminates with the laby- 

 rinth region — there is no occipital portion of the skull. 



The post-otic region of the Selachian head has been inves- 

 tigated by Neal, Fiirbringer, Van Wijhe, Rabl, Beard, 

 Sedgwick, C. K. Hoffman, Sewertzoff, Brans, Froriep, and 

 Dohrn. The chief results of their inquiries about this region 

 may be summarised as follows. 



There is present an occipital region of the cartilaginous 

 skull, lying behind the labyrinth region. This is penetrated 

 by a number of nerves, serially homologous with the spinal 

 nerves, and known as the spino-occipital nerves. These are 

 often three in number in the adult, but may vary from none 



