496 Development of Mouth and Gills in Bdellostoma 
that the two forward lateral prongs have, as we traced them, posteriorly 
fused with the middle copular portion, which is shown in the figure as 
the median ridge on the throat floor and as a ventral triangular protru- 
sion of the median mesoderm. The lateral prongs have also become joined 
to the ventral wall of the head. Four sections back of this one they 
join the head laterally also, and thus the vertical side spaces seen in the 
figure are lost, leaving only the upper space as the throat cavity. It is 
plainly seen that the entire mass of tissue ventral of the throat cavity 
and included in the forward mandibular arch is the tissue that 
should form a lower jaw in an embryo, and in no particular the tissue 
to form simply the tongue of an animal. Its extreme anterior origin 
is in itself strongly against such a view. 
Development continues without any marked points of interest until 
we examine an embryo of about 28 mm. in length. This may be de- 
scribed as follows: The mouth opening is almost formed, as the secondare 
Rachenhaut is beginning to disintegrate, the gills have shifted backward, 
being more than twice as far posterior of the auditory vesicles than 
these are from the anterior point of the head. The gills are well 
pouched and the cartilage arches or cuffs are forming about their external 
tubes. A large and well-formed club muscle extends from the velar 
region to the first gill, this organ only appears after the gills have shifted 
some distance back of the hyoid cleft. The circular muscle of the club 
first arises as two muscle bodies some distance apart and dorso-lateral 
to the longitudinal one, and as it develops the circular muscle migrates 
ventrally and surrounds the long one. 
This embryo has its dental-plate extending forward as two large 
prongs which may be traced a long way back before the median copular 
portion is reached. The prongs are thus seen to grow ahead of this 
piece in their forward development, for we recall how close to their 
anterior ends the copular portion was found in the embryo of Figs. 7 
and 8. The early anlage of the teeth now appear on the median faces 
of these two prongs. The teeth are here entirely derived from the 
epiblastic wall. Cartilages are developing one to each prong of the 
dental-plate and its substance is very vascular, enclosing large blood 
sinuses. Fig. 11 is a section through the anterior region of the dental- 
plate and shows two tentacles, T, cross sectioned below, while two others, 
T’, are cut through their base of attachment. The Rachenhaut is disin- 
tegrating along its middle portion. The cartilages of the two prongs are 
shown coarsely stippled and the numerous ‘blood vessels. By, are indi- 
cated in outline, two enormous sinuses being located one along each 
dorso-median border of the prongs. The teeth anlage are represented by 
