BILATERALITY IN CRYPTOBRANCHUS 367 



there is no fixed relation between the direction of the first cleavage 

 furrow and the median plane of the gastrula. Since a fairly- 

 definite relation has been estabhshed between the direction of first 

 cleavage and the direction in which the spermatozoon enters the 

 egg, it follows that there is no fixed relation between the entrance- 

 path of the spermatozoon and the median plane of the gastrula. 



C. The relation of the first cleavage furrow to the median plane of the 



gastrula 



Does the first cleavage furrow separate the material for right 

 and left halves, or for anterior and posterior ends of the embryo? 

 Beginning with the third cleavage stage, biradial symmetry is 

 a conspicuous feature of the superficial cleavage pattern, and in 

 the lower hemisphere serves as a ready means for identifying 

 the early cleavage furrows up to the gastrula stage (figs. 4 to 

 13, 26 and 27). Is the cleavage determinate, and is this biradial 

 synmietry a mode of expression of the definitive bilateral sym- 

 metry of the embryo? Since the axis of biradiality is marked by 

 the first cleavage furrow, in answering this question it is necessary 

 to consider only the relation of this furrow to the median plane of 

 the gastrula. 



1 . Orientation experiments. The question was first investigated 

 by the simple device of orienting, without removing the envelopes, 

 a large number of eggs in the first cleavage stage in such a manner 

 that the planes of first cleavage were parallel, and leaving them 

 in this position to develop to the gastrula stage. At the time of 

 first cleavage the egg naturally and freely orients itself within 

 the envelope in such fashion that the animal pole is kept upper- 

 most. This experiment was performed on several different 

 occasions, using in all more than a hundred eggs; nearly all sur- 

 vived to the gastrula stage. The results, which were recorded 

 by means of diagrams, fail to show any tendency for either the 

 first or the second cleavage furrow to coincide with the median 

 plane of the gastrula. It seems unnecessary to pubUsh the data, 

 since the point is conclusively established by the more exact 

 experiments that follow. 



