Charles R. Stockard 501 
The two pairs of post-hyomandibular or thyroidean gills, (neither 
name, unfortunately, designates them accurately) always remain in their 
original position, not being affected by the shifting process which carries 
the true branchial gills back into the trunk region. In this connection 
it is important to note that these two gills are always drawn well into 
the head region and never occur out on the lateral gill plates where 
all of the following ones first make their appearance. The thyroidean 
gills disappear soon after the shifting starts. . 
The young embryos in which the diverticula of the mandibular arch 
bend upward show these gills in the following condition. In Fig. 19, 
which passes through the first of these two gills, the ectoderm forms a 
depressed pocket and is much thickened where the gut diverticulum 
comes close to it, the endoderm also thickening. This is in the anterior 
auditory region. The second thyroidean gill is much the same and is 
found in the posterior auditory region, one thus following close after the 
other. The next gill, which is destined to form the first true branchial, 
follows at an equal distance behind the second and may be seen for 
comparison in Fig. 23. This is not so well-developed as the thyroidean 
gills, being further caudad in the series and the developmental stages are 
gradually less advanced as we pass tailward. 
In embryos which have the mandibular arch flattened horizontally 
and which show seven or eight gill clefts, the first thyroidean gill as 
shown in Fig. 20 has a well-marked ectodermal pocket and the cor- 
responding gut diverticula is greater in extent than that of any following 
gill. Fig. 24 shows the first branchial gill in the same embryo. It is 
situated just at the base of the laterally outspread gill plate. The first 
thyroidean gill is again in the anterior auditory region and the second 
stops just as the auditory vesicle fades out posteriorly. In Fig. 20 the 
slight nick, 2t, just ventral of the first thyroidean is the most anterior 
tip of the gut diverticulum for the second thyroidean; all of the gills 
having an obliquely caudal direction from pharynx to body wall. 
When the embryos are 15 mm. long (a stage in which the mandibular 
arch is as seen in Fig. 3 and the hyomandibular in Fig. 17) the first 
thyroidean gill is in the condition shown in Fig. 21. It is still in the 
anterior auditory region, but one will note that the ectodermal thicken- 
ings and pockets have now disappeared as have the hyomandibular ecto- 
dermal thickenings at this stage, Fig. 17. The gut diverticulum will 
be noticed now to extend little more than one-half of the distance from 
the gut to the ectodermal wall. The second thyroidean gill presents much 
the same appearance, the two are therefore undoubtedly degenerating 
