PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 625 



was interpreted by Brachet ('98) as a part of the heart (see 

 Muthmann, '04, fig. 31, which was taken from Brachet's fig. 8, 

 p. 34) his error being later corrected by Muthmann. 



The marginal limits of this anlage are not well defined. Figure 

 1 represents a section taken as nearly as possible through the 

 medial sagittal plane of the thyreoid anlage of this stage. On 

 either side of this section, the ventrally directed fold of entoderm 

 is less pronounced. Transversely, the anlage is cut in five suc- 

 cessive sections (10 micra each), the general appearance of the 

 anlage is shown in figure 2, and somewhat in more detail in 

 figure 3. On either side of the anlage, mesodermal cells are 

 developing the mandibular arteries, but there are no other 

 structures yet formed in this region. The approximate lateral 

 limits of the anlage in this stage are indicated by r;r. 



From the phylogenetic standpoint much interest attaches to 

 the question as to whether the early anlage of the thjTeoid in the 

 amphibia is solid or hollow. In Amblystoma, it is evidently solid, 

 and thus is similar to the structure found in Triton, Siredon, Sala- 

 mandra by Maurer ('88) ; Salamandrina by Livini ('02) : and 

 Triton by Muthmann ('04). Miss Piatt ('96), on the other 

 hand, claims that not all the urodeles have a solid thyreoid bud, 

 but some may have a hollow one, as was the case in Necturus 

 described by her. Marcus ('08), finds a hollow anlage in Hypo- 

 geophis, a Gymnophionian. For the anurans, contradictory 

 statements occur in the literature. In Bufo and Rana, Maurer 

 ('88) claims it is first vesicular and two days after hatching it 

 becomes solid, but W. Miiller ('71), observed a solid one in Rana 

 temporaria and R. platyrrhinus, in which the first lumen appeared 

 in 25 mm. larvae, after the gland had divided into halves. 



In Amblystoma, the cells of the solid distal end soon proliferate 

 and form an elongated cylindrical bud in the stage next to be 

 described. 



B. Amblystoma larvae, 8 mm. long. Before proceeding to the 

 account of the thyreoid anlage in this stage, certain shiftings due 

 to growth of the surrounding parts may be described. By 

 marked increase in size of the brain and spinal cord above; the 

 forward and upward growth of the pericardial chamber and its 



