482 Development of Mouth and Gills in Bdellostoma 
ing the jaws and branchial arches can now be upheld will be discussed in 
the present paper. 
Howes, gi, says, “ In view of the admitted importance of (the hypo- 
physis in Petromyzon) it testifies, to my mind, to an enormity in the 
gap between the Marsipobranchii and the remaining higher Vertebrata 
which even Balfour’s conclusion that the former are the remnants of a 
primitive group, and Haeckel’s famous aphorism that they are further 
removed from the fishes than are the fishes from man insufficiently ex- 
presses.” 
Max Fiirbringer, 00, regarding the relationship between the myxinoids 
and petromyzontes states, “In wesentlichen Verhaltnissen unterschieden 
sich die Myxinoiden mehr und principieller von den Petromyzonten, 
als z. B. die Selachier von den Saugethieren; in einzelnen Merkmalen 
stellten sie sich selbst weiter ab von den Petromyzonten, als diese von den 
Gnathostomen und zeigten zugleich mancherlei Hinneigungen nach den 
Akraniern.” 
A long list of other workers Dean, 99, Johnston, 05, and Worthington, 
05, have recognized in these animals very primitive vertebrates coming 
as it were between amphioxus and the true fishes, and. from a study 
of their embryology and morphology have attempted to arrive at the 
ancestral or primitive type of vertebrate organs. 
Ayers and Jackson, 00, with a few others, are, I think, nearest to the 
correct position in considering marsipobranchs neither as degenerate 
parasitic forms nor as altogether archaic types, but rather as primitive 
animals especially adapted to their peculiar hfe habits. 
Recognizing the chaotic condition of opinion regarding these animals 
Prof. Dean kindly suggested to me that I undertake a careful study of 
the development of the organs in the head of Bdellostoma with the 
hope that the general problem of myxinoid relationship might be more 
clearly understood. Von Kupffer had in 1900 published the results of 
a similar study which he made on scanty and defective material, as is 
repeatedly mentioned throughout his paper. His study was also confined 
to rather young stages and, as I have found, stopped really before the 
most interesting points in the development had been reached. 
I am under obligation to Professor Bashford Dean not only for 
suggesting this study and for his criticism during its progress, but 
also for his kindness in placing entirely at my disposal his unique series 
of Bdellostoma embryos. I am also glad to express my appreciation of 
the encouraging interest that Prof. T. H. Morgan has always taken in 
my work. 
