-5- 



These partitions or sopta are shov/n 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 al- 

 most medially and paspin.fj slightly to the side of the medial 

 line of the mouth invagination. The anterior septum ( a. h. I.) 

 is seen to be further advanced than the posterior one, which ic 

 just 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 vrall. That the section represented in 

 Fig, 2 did not cut the thyroid in an exact median plane ic shown 

 by the much elongated cells which form the floor of the froove. 

 The section was probably somewhat oblique. The body wall of 

 the embryo is shov/n in outline (b.w. ), and the first indication 

 of gill clefts is seen as tv/o or three evaginations of the i^ha- 

 rynx dorsal and anterior to the thyroid (v,c.), A transverse 

 section of the t"'elve-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), •vith thick walls, lying just beneath 

 the pharynx and between two large blood vessels. Throughout 

 the mesoblast that surrounds the thyroid are scattered niimerous 

 yolk granules, the mesoblast cells themselves being rounded 

 and very different in shape from what they will be in later 

 stages. The cells in the floor of the pharynx are not so sharp- 

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



