PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 627 



space above the thyreoid anlage and between it and the ventral 

 wall of the pharynx is an accumulation of numerous darkly stain- 

 ing mesodermal cells (co), which form the early anlage of the 

 copula of the branchial arches, while below the anlage or a little 

 to either side of it, close to the ventral surface of the body wall, 

 are the early anlagen of the genio- and mylo-hyoideus muscles. 

 (The last named structures are shown in transverse sections, see 

 figure 7, which is taken in the plane indicated by the line d-d of 

 figure 4.) 



Figure 6 is through the most caudal lateral extremity of the 

 hyomandibular pouch (taken in the plane indicated by the line 

 c-c of figure 4), and shows not only the thyreoid anlage {tr) 

 at its point of junction with the pharyngeal entoderm of the 

 middle line of the floor of the pharynx, but also the anlage of the 

 first thymus body as already described. The anlage of the genio- 

 hyoideus muscle is a mass of illy-defined cells at the lateral side of 

 the thyreoid, while the anlage of the mylohyoideus muscle (rn.hy) 

 is somewhat more plainly outlined immediately below it. 



Figure 7 is about 75 micra caudad of the one just described 

 (taken in the plane indicated by the line d-d of figure 4) and shows 

 the thyreoid in this region completely isolated from the pharynx. 

 A broad sheet of mesodermal cells to form the copula and bran- 

 chial arches is now interposed between the thyreoid anlage and 

 the pharyngeal floor, while below is the anlagen of the muscles, 

 which are much the same as in the other section. 



In this stage a differentiation in the appearance of the cells of 

 the different germ layers has begun. Those of the pharyngeal 

 wall and of the thyreoid outgrowth are still filled with yolk gran- 

 ules and are much lighter in color than those of the mesoderm 

 in which the yolk is rapidly being absorbed. This differentiation 

 is, however, more pronounced in the stages immediately fol- 

 lowing. Microscopically there is little difference between the 

 cells of the pharynx and those forming the distal region of the 

 thyreoid anlage, bothhaving large round or oval nuclei. 



C. Amblystoma larvae, 9.5 to 11 mm. long. This stage may be 

 characterized as the period of most rapid development, both for 



