Charles R. Stockard 497 
the thickened portions of the epiblast. Tracing the organs caudally 
through the series we find the two dental-plate bodies becoming joined 
to the head laterally and joining one another medio-ventrally until we 
reach a section, Fig. 12, where the lower surface of these dental- 
plate bodies forms the ventral wall of the head. ‘They are thus in all 
respects similar to a lower jaw in position and composition, and have as 
far as I am able to interpret nothing about them suggestive of a tongue. 
The median furrow left by the incomplete dorsal union of the two 
prongs runs for many sections through the throat region, then is lost 
as the thin middle body referred to above comes between the two 
halves of the throat floor and forms a slight ridge along the median 
line. The hypophysis, seen in Figs. 11 and 12, runs back for quite a 
distance before opening into the gut. 
As development progresses the two anterior parts of the dental-plate 
become attached laterally to the head further and further forward until 
they cease to project as free portions. Studying an embryo 43 mm. 
iong, in which the brain has flattened and become almost solid as in the 
adult, and in which the nasal opening is now anterior and terminal, 
instead of ventral as in younger stages, we find that the mouth is a 
longitudinal opening much as it is in the mature condition. This open- 
ing or longitudinal mouth is easily comparable with the ordinary trans- 
verse mouth of other vertebrates when we remember that in the myxin- 
oids the forward portions of the lower jaw, which are hardly more 
than lip folds, have remained separate in the old embryos and_ that 
the tooth bearing portion has been earried to the back of the mouth. 
Now if we should unite medianly the forward projections of the dental- 
plate, the mouth could only open as an anterior-ventral transverse 
slit, or a slit in front of the lower jaw border as it does in other verte- 
brates. Referring to Dean’s Fig. 113 of a total embryo, the anterior 
prongs of the dental-plate are indicated by shaded portions, and the 
lighter median streak shows their forward separation. ‘The figure makes 
clear the close similarity to a transverse mouth. Dean _ writes, 
p. 263, that, “ The mouth cavity suggests closely that of a gnathostome ; 
in ventral aspect it appears as a narrow crescent whose conyexity is 
directed forward.” 
The longitudinal condition of the mouth will be made clearer by 
examining sections of the 43 mm. embryo. Fig. 13 is through the mouth 
opening and the two forward lip-like folds of the dental-plate, If, are 
fused laterally with the head. One of the tentacles, T,, is 
sectioned below. As the two lip-like bodies are followed back through 
the series they are found to be continuous with the dental-plate of which 
36 
