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section of this embryo just posterior to the first gill pocket, Gl. 1 in 
the figure being the last portion of this gill to appear in the sections. 
On one side of the pharynx is seen the outpushing of the second gill 
tube, Gi. 2. From the floor of the pharynx the thyroid anlage grows 
downward as a ventral cell mass. Fig. 2 shows a section of the same 
embryo through the region of the fifth gill, Gl. 5. The head fold has 
not become complete so far back and the embryo is ventrally attached. 
Here also is shown the cell mass below the pharynx floor, the thyroid 
Fig. 1. A section through the posterior part of the first gill pouch, showing 
thyroid anlage below the pharynx. About 30 diameters. Bs. Blood sinus. Bv. Blood 
vessels. Ch. Chorda. Gil. 1. Portion of first gill. Gl. 2. Anterior part of second gill. 
K. Nephridia. M. Myomere. N. Nerve tube. P. Pharynx. Zh. Thyroid anlage. 
Fig. 2. A section through the fifth gill with thyroid anlage below the pharynx. 
Gl. 5. Fifth gill tube. Hf. Peculiarly thickened ectoderm of the head fold. Zh. Thyroid 
anlage. 7r. Truncus arteriosus or branchial aorta. 
anlage. Fig. 3 gives a series of eight sections taken from the various 
regions of the gill area that are indicated in the accompaning guide 
figure. By a study of such a series one may form a definite idea of 
the extent and general form of the groove-like outpushing from the 
pharynx which represents the earliest condition of the thyroid in the 
Myxinoid embryo. 
The many possibilities of error have been taken into consideration 
in the interpretation of this outpushing, such as the effect of an ap- 
parent evagination which might result from oblique sections through 
this region, and such sections might be readily gotten on account of 
the bent shape in most embryos due to their being wrapped about 
the large yolk mass. I am certain that the groove described above 
is never due to such a condition. The change in position-relation of 
