The origin and histogenesis of the thymus in Raja batis. 451 



the lamprey is in a far lower degree of development, and, possibly, in 

 a more primitive condition, than in any of the other fishes. 



Moreover, from the facts laid down above new light is thrown 

 upon VON Kupffer's masterly researches (1891 and 1895) into the de- 

 velopment of the cranial nerves etc. of the lamprey. Hitherto it has 

 not been possible to bring these into agreement with the known facts 

 of other cases. What has for years kept the writer back from a com- 

 parative investigation of these matters in the lamprey and skate, (for 

 gf the former his collected material goes up to young ammocoetes of 

 14 mm), has been the difficulty of understanding von Kupffer's spinal 

 nerves of the head and his mesoblastic somites. No light would come, 

 and not until the recent great set of Dohrn's "Studien" (1901) ap- 

 peared, was it clear, that in this respect also the lamprey represented 

 a more primitive condition than some Elasmobranchs ^). 



The lamprey is a difficult material, and it is perhaps too much 

 to expect the learned Munich anatomist to furnish a clear account of 

 all that happens in embryos earlier than those of 4 mm. Had he 

 been able to elucidate the facts here, I believe it would have been 

 seen, that the later double fusion of the chief portion of a cranial 

 ganglion with two placodes of epiblast, epibranchial and lateral respec- 

 tively, was preceded by a single one on the level of the notochord. 

 For, in fact, from von Kupffer's and Miss Alcock's researches, the 

 lamprey would appear to have retained, not only the set of sense 

 organs, derived from the dorsal portion of the placode, but also for 

 all the arches the series, formed from the ventral part of the placode. 

 In the skate and some other Elasmobranchs this ventral series is only 

 represented in the development by a ventral neuro-epithelial placode, 

 except in one arch, the hyoid, where the ventral series is retained as 

 the hyoid complex of sense organs. 



show, that in his view the Elasmobranch sense organs are no better 

 off than those of the lamprey, and that both are embryologically — 

 unknown! Comment is quite supex'fluous. 



1) Without wishing to anticipate the ending of the forthcoming 

 controversy between Dohkn and Froriep, it may be permitted me to 

 express the view, that the results and conclusions of the former re- 

 present an immense advance over those, enunciated by the latter in 

 three recent publications (1901 and 1902). The works of von Kupffer 

 and DoHHN bid fair to revolutionize our conceptions of the nature of 

 the vertebrate head. 



29* 



