Normal Plates of the Development of Necturus maculosus. 20 



Somites. 



In some embryos three myotomes are differentiated before the neural folds are closed, while in 

 others there are no traces of myotomes until the folds are closed. The number of myotomes early becomes 

 exceedingly variable in the tail. It is here impossible to count them accurately either in surface views or 

 in sections. In the trunk i. e. betvveen the limbs from the 15 — 16 mm larva up to the 39 mm they seem to 

 be fairly constant numbering 18—20. In the tail however they are variable, so that in larvae of identical 

 lengths there may be a Variation of 1—5 myotomes. It should be emphasized that the number in the tail 

 is determined with great difficulty since the most caudal are but slight thickenings in the mesoderm. 



Lateral curvature of body. 



In the embryos of 8 — 15 mm there is much Variation in the lateral curvature of the body. In some 

 rests as high as 80 7o of the embryos have the head and tail curved to the right. Other nests show a like 

 percentage in which the head and tail are curved to the left. Out of 328 eggs, taken from five nests, 174 

 had the concave side on the right and 154 on the left. 



External gills. 



In most embryos of 9 mm (Fig. 25) the fourth and fifth arches are a common mass with no indi> 

 cations of the line of division; in some the line of invagination is distinct. In the later stages (e. g. 25 mm) 

 there are usually five or six filaments on the middle gill bar, in others there are eight or nine. This 

 Variation is even more pronounced in the 26 — 39 mm larvae. 



Limbs. 



Some variations have been noted in the time of appearance of both the anterior and the posterior 

 limb buds. The anläge of the anterior limb is usually discernible in the 11 mm stage (Fig. 27), but 

 sometimes not until the larva measures 12 mm. The posterior limb buds are usually beginning in the 12 mm 

 stage, sometimes are not present until the larva measures 13 mm. In the formation of the digits variations 

 are found. Three are usually present on the anterior limbs in the 20—21 mm larva. In some three are 

 not present until the larva is 23 mm long. The same Variation is observed in the time of formation of the 

 fourth digit on the anterior limb. Similar variations are found in the time of formation of the posterior digits. 



Pigmentation. 



Although little Variation is observed in the position of the bands there is much Variation in the 

 degree of pigmentation. These differences are most pronounced in the larvae between 19—25 mm. In 

 some the chromatophores are densely aggregated while in others they are sparsely scattered. In some 

 (25 mm) they have extended over the dorsal portion of the yolk only, while in others they have extended 

 over one half of the lateral surface of the yolk. 



