EMBRYOLOGY OF CRYPTOBRANCHUS 527 



over a period of about a week, with a corresponding variation 

 in the degree of development of different individuals at the time 

 of escape from the envelopes. On the average, hatching occurs 

 about two weeks later than the beginning of Stage 22, and about 

 six weeks after fertilization. Previous to the hatching period, 

 the envelopes become much softened and considerably enlarged 

 by the absorption of water, making room for the growing embryo. 

 The latter usually escapes by pushing ('worming') its way through 

 the envelope, leaving a small round hole; in some cases it bursts 

 the envelope by means of wriggling movements. 



The newly hatched larva measures about 23 to 25 mm. in 

 length. Very noticeable is the retention of a large yolk sac with 

 conspicuous bright-red vitelline veins; the bushy external gills 

 are pink with blood. In proportion to size of body, the tail is 

 much larger than in the adult. The dorsal surface of the body 

 and the sides of the tail are well pigmented, but in general the 

 larva of Cryptobranchus, like that of Necturus, is pale as com- 

 pared with amphibian larvae that develop from a pigmented 

 egg exposed to the light. The ventral surface is lacking in pig- 

 ment, leaving the abdominal region yellow from the presence of 

 yolk, and the throat region transparent. The heart can be read- 

 ily observed without dissection. The anterior limb rudiment 

 is provided with two digits. In most specimens the body somites 

 are plainly visible, but they do not show well in the photographs. 

 On account of its large size, graceful outlines and bright colors, 

 the newly-hatched larva is a striking and beautiful object. 



In the resting position, the larva lies on its side, turning occa- 

 sionally from one side to the other. The newly hatched larva is 

 able to swim rapidly in a straight line for a short distance, using 

 the tail as a propeller. The larvae avoid the light, and are posi- 

 tively rheotactic. 



The vitelline veins (figs. 195, 196 and 261) have shifted further 

 toward the mid-ventral line ; on one side of the body these veins 

 are well developed, on the other side they show arrest of devel- 

 opment with signs of atrophy. The heart now beats about sixty 

 to seventy times per minute. 



