216 



Mr. W. K. Parker on the Structure and [Dec. 19, 



From familiar tilings I pass to things little known ; that is, to the 

 early stages of the lizard. 



In the early stages I cannot confine myself to the nerve- supporting 

 organs, but, of set purpose, let my work overlap that of my friend 

 Mr. Balfour, who is, to me, the typical embryologist ; Mr. Milnes 

 Marshall's excellent papers, however, are not forgotten. 



Much that is figured of the earlier stages is not described ; my 

 illustrations can, however, easily be compared with those of the 

 cliick in Foster and Balfour's work; and with the copious and exquisite 

 illustrations given in Mr. Balfour's work on the " Elasmobranchs." 



The reader is asked to refer to these works, especially the latter ; 

 that he may see how perfectly my observations on the embryo of 

 the lizard correspond with what Mr. Balfour has discovered in other 

 types. 



Some of the most important of them relate to structures that must 

 bs well understood before we can gain even the most elementary con- 

 ceptions of the morphology of the vertebrate skeleton. 



These are — the brain and main nerves ; the sense- capsules ; the 

 respiratory openings (clefts) through the wall of the throat ; the 

 " pituitary body," and its relation to the mouth and brain ; and the 

 extension into and subdivision of of the pleuro-peritoneal cavity in the 

 head, even in front of the mouth. 



The modification of the " segmental " muscular masses in the head ; 

 the difference between the axial structures of the head and the body ; 

 all these things have to be carefully attended to. 



I will now propound my own theory of the skeleton of the head 

 and throat, as compared with the skeleton of the body generally, 

 namely, the spine and thoracico-abdominal cavity. 



The undivided condition of the paired tracts, on each side of the 

 notochord, which is so constant in the head, is the original state of 

 things ; the head is archaic, the trunk, with its vertebras intercalating 

 with the muscle-plates, is a much more modern result of evolutional 

 metamorphosis than the undivided head ; the limb-girdles and limbs 

 are the newest of all. 



Archaic entomocranial Vertebrates, had no vertebras, properly speak- 

 ing ; they had a long head, composed of fourteen or fifteen segments ; 

 their throat was a large multip erf orate bag ; and instead of having 

 one vagus nerve, they had seven or eight pairs of vagi, forking over 

 all the respiratory passages, except those supplied by the glosso- 

 pharyngeal and portio dura. 



Some of them were like Csecilians ; they had long, vermiform 

 bodies, and scarcely any tail behind their anal opening; they had no 

 finished vertebras, but a semi-solid, half- cartilaginous tube, surrounding 

 the notochord. 



Others were a sort of exaggerated tadpoles ; they were the fathers 



