1902.] NATURAL SCIENCES OF PHILADELPHIA. 87 



Fig. le, though representing a slightly older stage, may be taken 

 to represent a transvei'se section through the middle of the gland 

 very shortly after its first appearance as an evagination from the 

 floor of the pharynx (phar.). The thick lateral walls and much 

 thinner floor of the thyroid are shown in this section. Anterior to 

 the point represented in this figure, the th>Toid groove is somewhat 

 wider, while posterior to this point it is slightly narrower and gradu- 

 ally becomes more and more shallow imtil it finally disappears. 



The thyroid at this stage, then, is a groove, relatively 

 larger, perhaps, than at any other time, and opening along its 

 entire length into the pharynx. 



(V2ih day). — On the twelfth day the thyroid begins to be shut 

 off from the pharynx by the growth of two horizontal partitions, 

 one roofing over the anterior end, the other roofing over the poste- 

 rior end, of the thyroid groove. 



These partitions or septa are shown, at a slightly later stage, in 

 fig. 3, which is a sagittal section through the anterior end of a 

 thirteen-day embryo, cutting the thyroid almost medially and pass- 

 ing slightly to the side of the medial line of the mouth invagina- 

 tion. The anterior septum (a. h. I. ) is seen to be further advanced 

 than the posterior one, which is ju^st beginning (p.h.l). In this 

 figure the cells of the thyroid evagination are stippled to distinguish 

 them from the remaining cells of the pharyngeal wall. That the 

 section represented in fig. 3 did not cut the thyroid in an exact 

 median plane is shown by the much elongated cells which form 

 the floor of the groove. The section was probably somewhat 

 oblique. The body wall of the embryo is shown in outline 

 (b.w.), and tlie first indication of gill clefts is seen as two or three 

 evaginations of the pharynx, dorsal and anterior to the thyroid 

 (v.c). A transvei-se section of the twelve-day embryo, through 

 the anterior end of the thyroid (through the line ab in fig. 3), 

 shows that it is now a closed cavity (fig. lb) with thick walls, 

 lying just beneath the pharynx and between two large blood ves- 

 sels. Throughout the mesoblast that surrounds the thyroid are 

 scattered numerous yolk granules, the mesoblast cells themselves 

 being rounded and very different in shape fx'om what they will be 

 in later stages. The cells in the floor of the phaiynx are not so 

 sharply differentiated from those in the roof of the thyroid as is 

 indicated in this figure. The cells of the body wall are very simi- 



