PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 675 



PLATE 1 



EXPLANATION OF FIGURES 



1. Medial sagittal section of 5mm. larva showing relation of the early thy- 

 reiod anlage (tr.) to the oral plate and the pericardium. (X 50) 



2. Transverse section on line a of figure 1, showing the thj^reoid anlage 

 (ir.) projecting between themandibular arteries of the two sides. (X 50) 



3. Enlarged portion of figure 2. fX 200) 



4. Medial sagittal section of 8mm. larva, showing the now elongated thyreoid 

 anlage and its relation to the ventral mesoderm (co) and the pericardium. X 50) 



5. Transverse section along line 6-6, of figure 4, showing cells of the stomo- 

 deum and the thickened oral plate. (X 50) 



6. Transverse section of 8 mm. larva, along line c-c, of figure 4; the anlage of 

 of the thyreoid (ir.) still connected with the pharynx. Anlage of first thj'mus 

 body (ti) is at the dorsal lateral angle of the hyomandibular pouch, and below the 

 facial ganglion (T7/). CX 50) 



7. Transverse section of 8 mm. larva, along line d-d, of figure 4, showing the 

 thyreoid anlage (tr. ) , ventral mesoderm ice.) and anlage of the geniohyoideus mus- 

 muscle (g.hy.) of either side. (X 50) 



8. Transverse section through the anlage of the second thymus body (tu) of 8 

 mm. larva. It arises from the entodermal cells of the dorsal angle of the first 

 branchial pouch (6r.,), and lies lateral and ventral to the glossopharyngeal gan- 

 glion (IX). (X 50) 



9. 10, 11 Transverse sections showing the anlagen of the third, fourth and fifth 

 thymus bodies of 8 mm. larva respectively (tm) (tiy) (ty)- (X 50). 



10a Transverse section of 8mm. larva, showing the anlage of the postl ran- 

 chial body at this stage. (X 200) 



12 Transverse section through the anlage of the first thymus body (^i) of 9.5 

 mm. larva. This section also shows the unpaired thyreoid (tr.) between the 

 genio-hyoid muscles; the copula lies just above it in the medial line. (X 50) 



13. Transverse section through the caudal extremity of the thyreoid of 9.5 

 mm. larva, showing its division into right and left halves in this region, the copula 

 dipping ventrally between the two. (X 50) 



14 Transverse section showing the second thymus body (tn) of 9.5 mm. larva 

 completely severed from the pharyngeal entoderm, and lying dorsal to the 

 pharynx, medial to the levator muscle of the second arch. (X 50) 



15 Transverse section showing the third thymus body (tiu) of 9.5 mm. larva 

 in connection with the pharyngeal epithelium in the region of the dorsal extremity 

 of the second branchial pouch. The anlage of the postbranchial body (p6.) is 

 also shown. (X 50) 



16, 17 Transverse sections showing the position of the fourth and fifth thymus 

 bodies (tiv) (tv) of 9.5 mm. larva in relation to their respective pharyngeal 

 pouches. (X 50) 



