5U0 



THE AMERICAN NATURALIST [Vol. XLIX 



Bufo, together with my own on AinUiiMoma, have thrown 

 doubt upon this conclusion. 



Still others (Newport, Roux) believe that the path of 

 the entering spermatozoon and the primary ovic axis 

 determine bilaterality. Jordan has shown that this view 

 is untenable for Diemyctylus. Professor Whitman's ob- 

 servations, recorded in a later paragraph, show that this 

 is not true in Bufo. 



Thus each of these assumptions has been met by 

 serious objections. 



The idea that the first plane of cleavage determines 

 the axis of the embryo was expressed as early as 1853 by 

 Newport in the following words: 



I have Ion- been aware that the axis of the embryo was in the line 

 of the first cleft of the yolk. 



From a series of experiments on the frog's egg Roux 

 came to the conclusion that the first cleavage plane coin- 

 cides with the median sagittal plane of the embryo. In 

 the same year Pfliiger reached the same conclusion. Sup- 

 ported by these eminent investigators the theory was very 

 generally accepted. In working over the same field 

 Rauber found that in the axolotl and frog the median 

 plane of the embryo coincided with the second cleavage 

 groove instead of the first. Shortly after the publica- 

 tion of Rauber 's work, 0. Hertwig working on the egg 

 of Triton confirmed the observations of Rauber. In 1892 

 Roux modified his earlier view and stated that the second 

 groove as well as the first often coincided with the median 

 plane of the embryo. 



In the following April the writer found from a series 

 of puncture experiments on the egg of A mhlystonxt that 

 exovates on opposite sides of the first cleavage groove 

 were later found on one side of the embryo. The conclu- 



not separate the right and left halves of the embryo. 



In 1893 Jordan and the writer reviewed the experi- 

 ments up to this date. We found that even in the de- 

 scriptions and figures given by Newport, Roux, Rauber, 

 there was evidence sufficient to show that the median 



