BILATERALITY IN CRYPTOBRANCHUS 361 



be disturbed. In the stages under consideration the egg or 

 embryo is devoid of ciUa; observation of landmarks furnished by 

 artificial markings show that during cleavage the eggs do not 

 undergo any perceptible rotation on a vertical axis. The inertia 

 of the heavy egg and contact of its lower surface with the sub- 

 stratum make it easy to guard against disturbances suffi.cient 

 to affect the position of the egg. In the first two or three sea- 

 sons' work, the Syracuse watch-glasses used for these experi- 

 ments were placed on a massive walnut table resting on a gravel 

 foundation and against a stone wall; in later experiments, com- 

 prising the greater part of the work, they were placed on the level 

 top of a concrete wall. To make up for the loss by evaporation, 

 water was each day added gently by means of a pipette. The 

 probability of error increases, of course, with the length of time 

 involved in the experiments. Wherever possible, the results 

 were checked by other methods. 



In certain experiments the eggs were removed from their gelat- 

 inous envelopes immediately after being taken from the uterus, 

 thereby allowing them to orient themselves, in water, at once 

 with the animal pole uppermost. This procedure eliminates, 

 during the fertilization period, the possible influence of gravity in 

 determining the direction of the median plane. The eggs used 

 in most of the experiments were taken from nests, where spawning 

 and fertilization took place in a natural manner; hence it was 

 necessary to test the possible influence of gravity under condi- 

 tions that sometimes occur in nature. 



A . The possible influence of gravity upon the direction of the median 



plane 



For this experiment eggs taken from the uterus of a ripe female 

 were artificially fertilized without removal from their envelopes. 

 Twenty-one eggs were placed each in a separate watch-glass 

 without water and oriented with their polar axes in a horizontal 

 position and parallel to each other; the animal pole was directed 

 away from the observer. After allowing time for the capsule to 

 adhere to the glass, a little water was added gently by means of a 

 pipette. In the course of one or two hours the capsules absorbed 



