EMBRYOLOGY OF CRYPTOBRANCHUS 537 



meres (e.g., through the egg becoming lodged in a crevice) might 

 in some cases result in the formation of a double embryo (Spe- 

 mann '01 to '03). But owing perhaps to the heavily yolk-laden 

 character of the egg, and the slowness with which the first furrow 

 cuts its way through the yolk, my attempts to produce a double 

 embryo in this manner have failed. When a fine silk thread or 

 a hair was tied about the egg in the two-cell stage so as to con- 

 strict it in the plane of the first cleavage furrow, the egg usually 

 burst before reaching the gastrula stage. In a single case the 

 egg lived until after the formation of the neural folds, but devel- 

 oped a single embryo with its principal axis at right angles to 

 the constricting cord. 



A more probable explanation is that in the two-cell stage the 

 egg became inverted and remained for some time in this position 

 subject to the rearrangement of contents through the disturbing 

 influence of gravity acting in a direction opposite to the normal 

 (Schultze '95). But the whole question is complicated by the 

 more fundamental problem of the determination of the median 

 plane of the embryo. My experiments, to be described in a later 

 paper, show that in Cryptobranchus the first cleavage furrow 

 tends to form at right angles to the direction of entrance of 

 the sperm (as in Triton, but not as in the frog where the first 

 cleavage tends to coincide in direction with the path of the sperm). 

 If in Cryptobranchus, as in the frog, the entrance path of the 

 sperm lies approximately in the median plane of the embryo, 

 then the conditions necessary for the production of a double 

 embryo by the separation of the first two blastomeres must be 

 of very exceptional occurrence. 



4. Spiral-tailed monsters 



In the fall of 1910, some embryos shipped by express to the 

 University of Wisconsin and reared there in city water, acquired, 

 in about 8 per cent of their number, the abnormality shown in 

 figures 201 and 202. From the condition of the tail in late larval 

 stages, specimens affected with this malformation may be desig- 

 nated as 'spiral-tailed monsters.' At first this peculiarity seemed 



JOURNAL OK MORPHOLOGY, VOL. 23, NO. .3 



