380 DAVIDSON BLACK. 



INTRODUCTORY 



The aims and scope of the present investigation have aheady 

 been set forth in my first communication on this subject (10), 

 of which this paper forms the second part. 



After the completion of Part I, I received a copy of a most 

 interesting thesis by H. Van der Horst (54). This author has 

 carried out a careful research upon the motor roots and nuclei 

 of the cerebral nerves, more especially upon the taxonomic 

 value of these structures in the lower vertebrates. Though his 

 extensive investigations in this field have been chiefly con- 

 cerned with the true fishes, yet in addition he has studied the 

 arrangement of the motor roots and nuclei of the brain stem in 

 two cyclostomes (viz., Bdellostoma dombeyi and Petromyzon 

 marinus) as well as in two amphibians (viz., Rana catesbeana 

 and Molge cristata). Charts of the motor nuclei and roots of 

 the cerebral nerves in all the forms studied have been recon- 

 structed by this author with great attention to detail and are 

 reproduced as figures I to LXXXI in his thesis. His descrip- 

 tion and chart of the motor nuclei and roots in Bdellostoma 

 dombeyi differ in no essential way from my own, and the same 

 may be said also with regard to his work on Selache maxima, 

 Polydon spathula and Ameiurus nebulosus. In the case of Rana 

 catesbeana the reconstruction chart by Van der Horst corresponds 

 closely with my own, though some slight differences appear and 

 will be noted subsequently in the present paper. Unfortu- 

 nately a detailed description of the motor roots and nuclei of 

 the cerebral nerves has been omitted by Van der Horst in the 

 case of the amphibians which he studied, so that his reconstruc- 

 tion charts only are available for comparison in the present 

 connection. 



The motor nuclei and roots have already been studied and 

 reconstruction charts have been made in the following anuran 

 amphibians: Rana esculenta (Kappers, 35); Rana fusca and 

 Bufo sp.? (Rothig, 50).^ These charts are reproduced in the 

 present paper in figure 9, p. 395. 



1 Rana fusca (Rosel), appears to be but another synonym for R. temporaria, 

 the common European Grass Frog (v. Gaupp, 25; Gadow, 22). Rothig does not 

 give the species of Bufo which he studied but it was most probably B. vulgaris. 



