504 Development of Mouth and Gills in Bdellostoma 
while the first branchial, seen in Fig. 25, has progressed considerably be- 
yond the condition shown in Fig. 24. 
Fig. 22 shows in an older embryo a stage of still further degeneration 
of the first thyroidean gill. Here again there is no ectodermal indication 
of the gill, and the gut diverticulum extends somewhat more than a 
third of the distance from the pharynx to the body wall. Finally in 
older embryos these gut diverticula become less and less important until 
they are lost entirely. Thus they cannot be found in a stage when the 
gills are beginning to form the pouches and have shifted back into the 
trunk region. 
Including the mandibular I have thus far described the developmental 
changes taking place in the four anterior pairs of gill clefts. The 
development of the mouth arch is progressive and highly modified; 
beginning as an ordinary gill cleft extending dorsally, it changes the 
position of its diverticula into a more and more ventral position, and 
finally undergoes complicated changes in connection with the develop- 
ment of the dental-plate. 
The three following gills, hyomandibular and the two thyroidean, 
undergo a distinct retrogressive development: in the young embryo they 
are the most advanced elements of the entire gill series; they then grad- 
ually degenerate and in late stages they become entirely lost. On the other 
hand, the first branchial cleft as we have seen develops progressively, 
and becomes the first true marsipobranch of the adult. All of the follow- 
ing gills progress in a similar manner and reach the mature condition in 
a way to be described below. 
THE DEVELOPMENT AND SHIFTING OF THE TRUE BRANCHIAL GILLS— 
PoucHEs, ARCHES, ETc. 
The development and changes in the relative position of the gills in 
Bdellostoma are among the most interesting phases in the embryology of 
a Myxinoid. Dean in his study of cleared embryos described the general 
processes of the gill development with remarkable correctness, so that 
one studying the finer details in serial sections is surprised at the manner 
in which the general points of development were interpreted from the 
total embryos. 
On page 261 Dean says, “In the hinder (head) region we have thus a 
view of the mode of closure of the fore-gut: and we note the drawing 
together of the sides of the gill-lappets and the mode of backward exten- 
tion of the ventral rim of the definitive pharynx, in a way which suggests 
curiously an analogy with reptiles and birds.” This description is just 
