4.92 Development of Mouth and Gills in Bdellostoma 
vagt is homologous with the hypoglossus nerve of the higher vertebrates.” 
As cited above in the general discussion Neal has shown that the an- 
terior segment of the M. parietalis ventralis is without relation to the 
tongue in either Ammocoetes or Petromyzon, while the muscles of the 
tongue of higher vertebrates are derived from this segment. I am sorry 
not to be able to give at first hand any statement regarding the so-called 
“tongue” of Petromyzon, but from a study of the hterature it seems 
that we are dealing here with an organ possibly as untongue-like as is the 
dental-plate of Bdellostoma. Gdppert says regarding the myxinoids, 
that since they fail to have a larval form the tongue is found arising 
here much earlier than in the petromyzontes. At the termination of 
embryonic development the tongue has attained its final form and 
functions from then on as the boring apparatus. He states that Kupffer, 
99 and oo, described and figured the massive anlage of the tongue in 
Bdellostoma as arising from the mesodermal tissue in a stage when the 
“ secondire Rachenhaut ” still existed. G6ppert concludes with the per- 
tinent remark that the cyclostome tongue remains a peculiar structure 
in contrast to the more simple tongue of the gnathostome fishes. 
Dean, 99, p. 269, refers but briefiy to the tongue, stating merely, that 
below the opening of the hypophysis into the gut the tongue arises 
on the ventral wall of the throat as a paired outgrowth, and its rapid 
development greatly modifies the shape of the mouth eavity. 
According to my more detailed studies the stages in the growth of the 
tongue are as follows: The earhest anlage of the dental-plate, as I shall 
term it from now on, is seen in Figs. 3 and 4 to be represented merely 
by the thickening of the mesodermal tissue in the ventral mouth region. 
This tissue composes the entire body of the embryonic head below the 
throat floor just as the lower jaw tissue does in embryos of Triton, 
Figs. 5 and 6, Cestracion, and other lower vertebrates. This mesoder- 
mal mass continues to increase in size and to become thicker in a dorso- 
ventral direction, while at the same time its anterior end becomes free 
from the lower body wall and divides into two distinct prongs or for- 
ward extensions, Fig. 7 lh. It will be seen that these prongs a short 
way back from their anterior tips come together in a thin vertical 
piece, Fig. 7 ep, which now alone connects them with the ventral wall 
of the head. It may be remarked that Fig. 7 is taken from an embryo 
in which the first four branchial gills have been drawn in to the head 
region. In Fig. 8, which is a section through the same embryo taken 
further tailward, we see the two lateral horns, lh, joined to the median 
portion, cp, and on the right side of the figure, the section being slightly 
oblique, the lateral prong has also joined the ventral body wall. Here 
