382 The American Naturalist. [May, 



ually found separate the anterior from the posterior part and 

 thus represent the normal second plane. Raubcr it is to be 

 noted, found somewhat similar irregularities. Thus in the frog 

 the first plane in seven eggs made the following angles with 

 the future median plane of the adult, 90°, 50°, 90, 85°, 0°, 

 82°, 90°. In the axolotl the same angles were, 80°, 53°, 90°, 

 50°, 90°, 30°, 90°, 2°, 90°, 90°, 70°, 80°, 32°, 90°, 90° in fifteen 

 eggs observed. 



Another point that appears to offer unusual difficulties to the 

 experimentator is connected with the movement the egg per- 

 forms after the embryo begins to form, which render the refer- 

 ence of organs to special regions of the egg by no means easy. 

 Thus Roux 3 -*' 5 and Schultze working upon similar material by 

 similar methods arrive at very different conceptions of the re- 

 lationships of the dorsal and ventral parts of the frog to the 

 white and black parts of the egg. 



Roux holds that the medullary folds, that is, the dorsal re- 

 gion of frog, appear upon the vvliit*- <»r lower pole. This is 

 seen when eggs are fastened in normal positions and is also 

 inferred from experiments in which injury to the black pole 

 remains as injury to the ventral sideot'the embryo. The reason 

 other observers find the doreal area on the upper black pole is 

 that the egg turns over so that the dorsal field floats upper- 

 most. The blastopore first appears just beneath the union of 

 black and white, the equator, and then shifts over while closing 

 to the opposite side of the equator. Where the blastopore first 



of blastopore ris 

 pole where the 1 



