438 



Fore-brain — 2, and possibly a portion of a third. 



Mid-brain — 2 or 3. 



Hind-brain — 6 or 5. 

 As regards the origin of the cranial nerves I can fully confirm the 

 observations of Orr, except as to the origin of the VI from the most 

 anterior neuromere of the hind brain. This point is somewhat obscure. 



General conclusions. 



I consider that the primitive vertebrate brain consisted of a series 

 of segments, similar to those found in the Embryonic spinal cord, and 

 that the encephalomeres probably held the same relation to the meso- 

 blastic head segments as the myelomeres do to their respective meso- 

 meres, that is they were intersomitic , the centre of each neuromere 

 being opposite the space between two somites and giving off a mixed 

 nerve from the apex. 



The region known as the Encephalon is the result of a great dif- 

 ferentiation and specialization of the anterior segments of this primi- 

 tive structure. That differentiation first began and has been the greatest 

 in the most anterior segments, which may account for the greater size 

 of the folds in this region than in the hind brain, which, less differen- 

 tiation and specialization having taken place, naturally conforms more 

 to the primitive vertebrate type. I am aware that the forms examined 

 are insufficient to enable us to reach any positive conclusion in regard 

 to the number of segments, but I feel confident that the method which 

 I have adopted is the one by which this vexed question of the primi- 

 tive segmentation of the head region, both of the neural tube and sur- 

 rounding mesoblast, will be eventually decided. 



In conclusion I may say that I feel confident that the full number 

 of primitive encephalomeres will be found in Elasmobranch, Ganoid or 

 Teleost embryos, the investigation of which will form the second part 

 of this paper. 



Princeton, June Hth, 1889. 



3. Hyalophyllum Haeckel = Copilia Dana cf. 



Von ]Jr. "VV. Giesbrecht, Neapel. 



eingeg. 29. Juli 1889. 



Das Corycaeiden-Genus Copilia und mehr noch das Genus Sap- 

 phirinella^ oder wie es später von Ha^ckel genannt wurde, Hyalo- 

 phylhim, ist aus den anatomischen und histologischen Untersuchungen, 

 welche Forscher wie Leuckart, Hwckel. Gegenbaur, Claus, 

 Grenacher, an diesen ungemein durchsichtigen Thieren anstellten, 



