No. 2.] EMBRYOLOGY OF THE LIZARD. 353 



chiefly to be tried. Hjibrechl,^ however, made vahiablc efforts 

 in this direction, and indicated some of the possibilities. 



The Nemertina have afforded the best basis for the application 

 of the theory of the paired origin of the ventral nerve-cord, and 

 also of the dorsal nerve-cord. Gegcnbmir'^ says: " Although in 

 most of them the longitudinal trunks run along the lateral edge 

 of the body (imbedded within the muscular layers), in others 

 (Oerstedia) they approach one another ventrally, and are dis- 

 tinguished by swellings at the joints, where nerve-branches are 

 given off. This is in anticipation of the future development of 

 ventral ganglia, the elements of which are already present in 

 the longitudinal trunks. The ventral approximation of the 

 longitudinal trunks shows us how the central nervous system 

 got its ventral position, which becomes further developed by 

 the formation of ganglia." HitbrecJit"^ has found that there is 

 a " sheath of ganglion-cells which uninteruptedly accompanies 

 these trunks from their origin in the cephalic lobes down to the 

 extremity of the tail in all genera without exception." Hnbrecht 

 has given reasons for considering the genus Carinella the most 

 primitive of the nemertine group. Of the lateral nerve in this 

 genus he says : " It is not surrounded by nerve-cells, as these 

 form only an external coating to it. . . . This cellular portion in 

 Carinella is also of a less compact nature than in those of more 

 differentiated genera, and is everywhere in direct contact with 

 the epidermoidal tissue." Here, then, we find a general resem- 

 blance to certain forms of the embryological development of 

 the vertebrate medulla, in which each lateral half shows in 

 cross-section a lens-shaped swelling of epidermoidal cells, on 

 the inner surface of which, later, the longitudinal nerve-fibres 

 develop. A glance at several different forms will make this 

 clearer. Balfotir (Comp. Embryol.) has given a figure, after 

 Kowalevsky, of an amphioxus larva, in which the medullary 

 plate is distinctly thinner along the median line. The same 

 author (Elasmob. Fishes) has pictured the median thinner 

 portion of the medullary plate of elasmobranch embryos with 



' See HnbrecJWs papers in recent volumes of the Quarf. yourn. of Alic. Sci. 



^ Gegenbaur. — Elements of Comparative Anatomy. Translat. 



^ Hubrecht. — Zur Anat. und Physiol, des Nervensystems der Nemertinen. Kon. 

 Akad. d. Wiss. Amsterdam — Researches on the Nervous System of Nemertines. 

 •^ Quart. Jour, of Alic. Sci., 1880. 



