486 Development of Mouth and Gills in Bdellostoma 
unrepresented in the higher vertebrata, the question how far the above- 
named median ventral cartilage may be comparable to the basi-mandi- 
bulo-hyo-branchials of the latter, as Miiller, Huxley, and Parker have 
together sought to show, must remain in abeyance, until more is known 
of the development of these fishes.” 
Howes considers the cartilage which Ayers and Jackson have called 
the cornual to be closely similar to that which Huxley called the Meckel’s 
cartilage in the lamprey and says “ with that I hold it to be homologous 
notwithstanding Parker’s view to the contrary.” 
On this point I must 
disagree with Howes and accept Ayers and Jackson’s idea regarding this 
cartilage, and further with them I entirely agree that the lower jaw struc- 
tures are to be found in the so-called “tongue” or dental-plate. This 
position, as I shall endeavor to show in the following pages is admirably 
supported by embryological study. 
2. As far as Petromyzon is concerned the development of the head 
has been studied by Kupffer, 94 and 95, Dohrn, 83, and others, although 
it appears that none of these authors have given particular attention to 
the peculiar development of the “tongue.” Kupffer, 99 and 00, also 
studied the head of Bdellostoma and discussed in detail the development 
of the mouth, finding as he claims a first or primitive mouth, which 
is transverse and leads directly back into the gut in a manner similar 
to the ordinary fish mouth. This primary mouth then becomes closed 
by a growth of the ectoderm, which Kupffer has ealled “ sekundire Rach- 
enhaut” the closure persisting until about the time of hatching when 
the wall is again broken to produce the final or secondary mouth. Max 
Fiirbringer has from this description and for the further reason that 
the hypophysis always opens into the throat, called the myxinoids Dis- 
toma, the petromyzontes Cyclostoma, and the higher vertebrates Gnatho- 
stoma. 
The general embryology of Bdellostoma has been beautifully worked 
out by Dean, 99, but he enters only in a general way into a discussion of 
the head organs. He gives a very correct account of the gill develop- 
ment from a study of cleared total embryos, the finer details being, of 
course, impossible to determine in this way. Dean stated that the gills 
were shifted during development from their original position close after 
the hyoid arch to their final adult place far back along the sides of 
the body. He also made the remarkable observation, considering his 
study to be upon cleared totals, that there is a “doubtful” cleft which 
is early suppressed lying close behind the hyomandibular, therefore cor- 
responding to the “ thyroidean” cleft of Dohrn. Dean also, very cor- 
