Charles R. Stockard 505 
such as one would give after a study of sections and when. the large 
yolk-bearing egg of Bdellostoma is recalled, resembling so closely the egos 
of reptiles and birds, we would expect the mechanics of development to 
operate in a comparable fashion, e. g., the closing together of the gut 
walls. We find on page 264 in a discussion of late embryos, that a great 
increase in the length of the neck region has taken place, the head now 
advancing around the anterior end of the egg. “ With this continued 
growth the further change in the position of the gill pouches is naturally 
connected. Contrasted with the preceding stage the interval between 
the row of gill pouches and the eye has become nearly doubled and in this 
space the tongue muscle has taken its position. The more rapid growth 
of the dorsal region of the head and neck, with the accompanying growth 
of the tongue muscle is, I believe, sufficient to account for the apparent 
translocation of the line of gills.” We shall later see how very close 
to such an explanation of the gill shifting we are forced after a careful 
consideration of the evidence gathered from a study of sections. By 
the “tongue muscle” Dean refers to the large “club muscle” arrange- 
ment of Ayers which les between the end of the velum and the first 
branchial gill and serves to operate the dental-plate during its vigorous 
feeding motions. 
I fully agree with Dean that, “Both Price and von Kupffer have 
assumed a correspondence between branchiomery and myomery of which 
in this form at least I can find no evidence.” Since Price’s, 96, idea, 
which he has corrected, 04, that a large number of gills disappear during 
development was erroneous the considerations of Ayers and Jackson, 
oo, and Johnston, 05, over this point must be disregarded or modified so 
as to apply to the true conditions. 
Development of the individual gill—Nothing prior to this has’ been 
contributed to the development of the Myxinoid gill, the formation of 
its complex pouch or the cartilaginous portions accompanying it. 
The entire series of gills develop alike with the exception of the 
simple cesophago-cutaneous duct which fails to form the pouch and has 
its external tube leading out in common with the gill next to it. The 
most anterior gill may be taken as an example since it is better developed 
than the following ones throughout almost the entire embryonic life. 
The earliest anlage of this gill consists of a slight thickening of the 
ectoderm and a corresponding thickening and uppushing of the endo- 
derm, the gut endoderm at this stage being spread out over the yolk, as 
seen in Fig. 23. As development progresses, Fig. 24, the ectodermal 
pocket becomes a fold just at the base of the lateral gill lappets and the 
