628 FRANCIS M. BALDWIN 



the general structures, and for the thyreoid anlage. The mouth 

 is now open except in the 9.5 mm. larvae; the gill pouches have 

 fused with the ectoderm, and the slits are open. The ventral 

 mesoderm forms well defined structures — the branchial arches, 

 and blood vessels — in the region below the pharyngeal tube. 



The anlage of the thyreoid in the 9.5 mm. larvae, is slightly 

 longer (about 140 micra) than that in the 8 mm. larvae, the 

 proximal end is still connected with the pharyngeal entoderm, 

 while the distal end is divided for about 40 micra into right and 

 left halves, thus giving to the whole a Y-shaped appearance, with 

 the forks of the Y much shortened. Figure 13 is a section through 

 the caudal region of the anlage in this stage, the right and left 

 halves lying just dorsal to the geniohyoid muscle. In the region 

 above the thyreoid anlage, the mesodermal' cells plainl}^ show 

 the relative positions of the anlagen of the copula (co) in the 

 medial region, and the hyoid {hy) the and first branchial arch 

 (c.hr.l), the relations of which will be detailed in 10 mm. larvae, 

 below. The inferior jugular vein lies a little lateral to the thy- 

 reoid anlage, while the external carotid artery is smaller and is 

 latero-ventral to the jugular. 



In the 10 mm. larvae the relationships differ considerably from 

 those just described. The thyreoid anlage has severed its con- 

 nection with the floor of the pharynx, and in addition is divided 

 completely into right and left halves, which assume a position a 

 little lateral to the geniohyoideus muscle of either side, the ce- 

 phalic ends of each being about in the plane with the junction of 

 the first branchial arch with the copula. The relative positions 

 of the thyreoid anlage and the developing copula and branchial 

 cartilages in the present and previous stages are the result of 

 differential growth. The thyreoid anlage is carried backward 

 below the copula, the keel of undifferentiated tissue extending 

 ventrally from the copula (possibly the anlage of the external 

 rectus muscle of the hyoid or the first branchial cartilages) inter- 

 vening between the halves. Maurer ('88, p. 360), in describing 

 this stage in Triton says, 



Dann (two or three days after hatching) beginnt die Differenzirung 

 des knorpeligen Kiemenapparates und die Kiemenspaltem offnen sich. 



