510 Development of Mouth and Gills in Bdellostoma 
of the branchial organs. My study of the sections has enabled me to 
add a few further details. 
In young stages where the gills are on the laterally outspread gill 
lappets as illustrated in the diagrams Figs. 33 and 34 no shifting at all 
has taken place. Each gill follows the preceding one at approximately 
equal intervals. In diagram 33 M indicates the horizontal outline of the 
mandibular arch, Hm the same for the hyomandibular cleft, and just 
following this are the two pair of thyroidean gills. The last, it will be 
seen, are well within the head region, and are indeed at no time on the 
lateral gill lappets. After the thyroidean gills, tg, (which are now more 
advanced in development than any of the true branchial ones) the other 
gill folds come in regular succession. The auditory vesicles are just 
over the first thyroidean gills. 
In more advanced embryos the gills are being folded in along the 
sides of the body as the gill lappets are drawn together, Fig. 34 shows 
the first few already brought in. The ear vesicles have increased in 
size and now lhe over the two thyroidean gills. When the gills are all 
just about folded in along the neck, a region of rapid growth is formed 
between the second thyroidean and the first branchial, or in other words 
just anterior of what was the forward limit of the gill lappets, or at the 
point marked x in Figs. 33, 34, and 35. This rapid growth of the dorsal 
region of the head and neck causes the head to advance around the 
anterior end of the egg, leaving the gills as it were behind. Later the 
large club muscle of the dental-plate begins to develop in the ventral neck 
region between the thyroidean and the first branchial gills, thus further 
facilitating the lengthening of this area and helping to cause the trans- 
location of the gills from their original place in the neck to their final 
position along the sides of the trunk. 
The myotomes in young stages begin behind the gills, the most anterior 
one being posterior of the last gill, but during development many myo- 
tomes come to lie anterior of the first gill. I find no definite relation 
between myotome and gill arrangement at any stage. Thus one must 
conclude from a study of Bdellostoma embryos that there is no essential 
correspondence between myomery and branchiomery. 
The exact growth point in the shifting process can be located since the 
second thyroidean gill undergoes its entire degeneration without changing 
its relative position, while the next gill which followed it so closely comes 
finally to lie a long way posterior to its place of origin. Since also the 
second thyroidean is the most posterior cleft which does not arise on the 
lateral gill lappet and the first branchial is the first one of the lappet 
series the idea at once suggests itself that the lappet area may be as it 
