561 



Fig. 2 passes through the posterior end of the oral fusion outlined 

 in fig. 1. The plane of section in figs. 2—5 is not true, and the 

 right of the figures is somewhat posterior to the left. In the plane 

 of fig. 2, the hyoid mesoderm has united with the mandibular on the 

 right, but is still separate on the left. An outward projection of the 

 median endoderm between the hyobranchial cleft and the oral fusion 

 of ectoderm and endoderm, is the base of the hyomaudibular pocket, 

 which has just passed out of section at the right. The small oval 

 space median to this endodermic projection is the cavity of the 

 alimentary canal continued into the thyroid outgrowth. The section 

 consequently passes through the beginning of this outgrowth, which 

 is thus seen to lie dorsal to the posterior extremity of the oral fusion 

 and to extend from the base of the hyomandibular pockets. 



Fig. 3 is four sections posterior to fig. 2. The mesoderm of the 

 hyoid and mandibular arches at the left is still separate. The 

 connection of the alimentary canal with the oral ectoderm has passed 

 out of section, and the thyroid outgrowth is found as a distinct oval 

 mass of cells, median to the branchial mesoderm, and bounded dorsally 

 by the ventral aorta, in which the anterior pair of aortic arches 

 sectioned in fig. 2 have united. 



In fig. 4, four sections posterior to fig. 3, the united hyoid 

 and mandibular mesoderm from each side of the embryo is about to 

 fuse in the median plane as shown by one or two mesoderm cells 

 which now unite these lateral tissues. Above the median mesoderm 

 cells, and apparently forming with them a continuous tissue, are the 

 posterior cells of the thyroid outgrowth, still bounded dorsally by 

 the large vascular space of the ventral aorta. In fig. 5, four sections 

 posterior to fig. 4, the thyroid outgrowth has disappeared, and in 

 place of the hyoid and mandibular mesoderm we find median 

 mesothelial tissue enclosing a si)ace continuous with the coelom, or 

 pericardial cavity. 



Figs. 2 — 5, therefore, show that the thyroid outgrowth arises 

 from the base of the hyomandibular pockets, directly above the oral 

 fusion, and extends backwards below the ventral aorta to the point 

 where the mesoderm of the hyoid and mandibular arches unites in 

 the median plane. As the embryo grows, the lateral extensions of 

 the oral fusion elongate, while the union of the tissues of the 

 mandibular arches in the median plane extends forwards , and the 

 longitudinal part of the oral fusion becomes obliterated. Meantime 

 the ventral aorta becomes enclosed in the pericardium and forms the 

 heart. The union of the thyroid outgrowth with the median mesoderm 



