Genital System of the tAyxinoidea 99 



Miinchen University, used the embryos obtained by Doflein for studying the develop' 

 ment of the head of Bdellostoma stouti (1899, 1900', 1906). He did not, however, express 

 any opinion as to the descent and relationships of the Myxinoidea. He believed the nose 

 developed from an unpaired "Reichplakode" (1900, p. 11), whereas Dean (1899, p. 273) 

 was of the opinion "that the embryonic nasal sacs arise from the early 'Nasenrachengang' 

 as distinctly paired structures." The adult nose shows no indication of ever having been 

 paired; but a paired nose in the embryonic stages of development would link the Myxi' 

 noidea with the Gnathostomes. 



Stockard (1906.2) wrote an excellent paper on the development of the mouth and 

 gills in Bdellostoma stouti, using Dean's collection of embryos for this purpose. He con' 

 eluded that the myxinoids are gnathostomatous vertebrates, the so'called ''tongue" 

 being the homologue of the gnathostome jaw. Furthermore he considered the myxinoids 

 as "undoubtedly primitive," but decidedly specialised in some respects. Neumayer (1910), 

 however, in a short discussion on the development of the cranium of Bdellostoma stouti 

 based on a study of a few embryos from Dean's collection, expressed the opinion that the 

 complete absence of an articulating jaw and the presence of an unpaired nose separate 

 the myxinoids from the Gnathostomes. 



My investigations on the development of the brain of Bdellostoma stouti (Conel, 

 1929, 1931) based on Dean's collection of embryos, lead me* to the conclusion that the 

 myxinoids are primitive, but, like all living vertebrates, have undergone some modifica- 

 tions as to structure. In early embryonic stages the brain follows the developmental 

 changes which occur in the ontogeny of all generalised vertebrates, at one stage resembling 

 especially closely the brain of J^iecturus. The eye is somewhat modified in the adult, 

 but develops in the manner typical of vertebrates. After an extensive and excellent study 

 of the fiber tracts in the adult brain of Myxine glutinosa, Jansen (1930) concluded that 

 "in spite of striking morphological differences, an analysis of the functional relations of the 

 nuclear masses in the brain of Myxine reveals great concordance with the conditions in 

 generalised vertebrate brains, like those of petromysonts and Amphibia." As a result of 

 these studies upon the development and adult structure of the brain, it seems reasonable 

 to conclude that the Myxinoidea, in respect to this organ at least, are primitive. 



As Dean (1899, p. 222) suggested might sometime occur, paleontology also has 

 recently furnished an important contribution toward solving the problem of descent and 

 relationships of the Myxinoidea. Fossil remains of Ostracoderms (Cyclostomes) have 

 been discovered in Spitsbergen by Norwegian expeditions from 1906 to 1925, and have 

 been excellently described and discussed by Stensio (1927). His studies upon these fossils 

 led him to the following conclusions (1927, p. 373): 



Thus the Ostracoderms had already very early separated into two main branches, one of 

 which includes the Osteostraci, Anaspida and Petromysontia, while the other consists of the 

 Heterostraci and Myxinoidea. To make it easy to refer to these two branches it will of course 

 be necessary to name them, and I therefore propose for the first mentioned branch the term 



