632 FRANCIS M. BALDWIN 



('11), in the teleosts have shown that the second stage may be 

 suppressed or absent. In the trout, Maurer ('86) says that the 

 formation of foUicles is very simple; soHd buds form on the pri- 

 mary vesicle (which is here hollow instead of solid as in the 

 urodeles), central cavities soon form in these, each then separates. 

 L. Muller ('96), believes that new follicles originate in the teleosts 

 from the older as buds from the epithelium which subsequently 

 become free. Gudernatsch ('11, p. 721) says, 



The reptiles, birds and mammals are said by the majority of observers 

 to show a vesicular thyreoid anlage, which changes into a compact organ 

 from which follicles later originate. Kolliker, however, observed in the 

 rabbit a thickening in the ventral wall of the pharynx from which a 

 wart-like solid process was cut off. Born also records the same for the 

 pig- 

 It would seem, from facts recorded above, that the formation 

 of the follicles in Amblystema differs slightly from Maurer's 

 account of the urodeles he studied (Triton, Salamandra atra and 

 Siredon), in that the solid cylinder of epithelial cells forming the 

 thyreoid an age (right and left) breaks up into groups of cells 

 which become scattered loosely about the wall of the inferior 

 jugular vein, or in the connective tissue of the region, and subse- 

 quently these isolated cells increase in number by mitosis and 

 form the walls of the follicles. Although Miss Piatt, ('96) in her 

 description of the development of the thyreoid of Necturus does 

 not enter the discussion of this point, it is very evident that she 

 noted a similar condition when she says, 



One finds in place of the solid thyrf^oid outgrowth (17 to 18 mm. lar- 

 vae), two lines of cells extending obliquely outwards . . . forming 

 neither a solid mass, nor walls of closed vesicles (follicles). These cells 

 of the thyreoid migrat? dorsal wards . . distinguished in an embryo of 

 19 mm. as a group of cells lying on each side, between the lateral surface 

 of the sternohyoideus. . . In an embryo 21 mm. one or two small 

 vesicles (follicles) are found where the group of cells (th.) lie in figure 8. 

 These vesicles increase in number as the embryo grows until in a little 

 amphibian of 46 mm., they constitute a considerable mass of vesicular 

 tissue. 



The relative position of these cells groups (which we may now 

 call follicles) differ not only in individuals, but on the two sides of 



