514 Development of Mouth and Gills in Bdellostoma 
a. Its earliest anlage comprises the entire mass of tissue ventral of 
the mandibular cleft, and from similar tissue in all other gnathostomes 
the elements of the lower jaw are differentiated during development. 
b. The “ tongue ” should better be called dental-plate since its muscles 
are not comparable with the tongue muscles of other vertebrates, and since 
it is innervated by the true lower jaw nerves, r. mandibularis of the 
trigeminus, as shown by many workers. 
e. The dental-plate originates too far forward in the embryo to be a 
tongue, it is not associated with the hyobranchial apparatus, and its 
bilateral manner of development with its paired cartilage is jaw-like. 
d. The development of the horny teeth on its surface and many other 
features of its later growth are jaw-like. 
e. The ventral wall of the dental-plate of old embryos forms also the 
ventral wall of the head; and the later development of this organ makes 
it clear that the longitudinal mouth of myxinoids is only a slightly 
altered transverse mouth. 
3. The hyomandibular cleft is enormously developed in young embryos, 
having well-formed ectodermal pockets in direct contact with the endo- 
dermal diverticula. A retrogressive development soon commences and 
the gut diverticula gradually decrease in size and finally disappear. 
Similarly all traces of the associated ectodermal pockets are finally lost. 
4. Two post hyomandibular gills occur in the early embryos and are 
for some time in advance of the true branchial gills in their state of de- 
velopment. They then begin to degenerate, and rapidly disappear. One 
of these gill pairs may correspond to the thyroidean gill of Dohrn. They 
‘are interesting since the maximum number of gills in chordates appears 
thus increased from fifteen to seventeen. 
5. The large and complex gill pouches of the adult are derived from 
the simple endodermal gill tube of the embryo by a spreading apart and 
subsequent folding of its walls in the region near its communication 
with the pharynx. 
6. The cartilaginous branchial arches of the embryo are scarcely dif- 
ferent from those of the adult. There are reasons for believing that the 
branchial skeleton of the Marsipobranchii is not essentially different 
from that of fishes: the extra-branchial condition is not entire and is 
probably secondary. 
7. A great change in relative position appears during the develop- 
ment of the gills. The gills originate in the posterior head or neck-region 
of the embryo and come finally to lie far back along the sides of the trunk. 
A region of rapid growth can be located between the second thyroidean 
