THE ORIGIN OF BILATERALITY IN 

 VERTEBRATES 1 



Professor A. C. EYCLESHYMER 

 Department of Anatomy, University of Illinois 



Many attempts have been made to determine liow early 

 in development the vertebrate egg becomes bilaterally 

 symmetrical. The conclusions have been as varied as the 

 attempts. 



Before the subject can be discussed it is necessary to 

 consider two fundamental propositions. The first is that 

 there exists an active pole in the egg, and the second is 

 that the anterior end of the embryo develops in this 

 region, or at least in the active hemisphere. 



The active pole is indicated at an early period by cer- 

 tain phenomena, such as secretory activity, accelerated 

 yolk metabolism, formation of pigment, position of 

 nucleus, expulsion of polar bodies, etc. Hatschek says 

 that 1 'it is probable that a polar differentiation is present 

 in the unfertilized ova of all the metazoa, through which 

 the most active and least active poles can be determined." 

 Whether or not Hatschek 's statement be true, it is certain 

 that if the area in which cleavage grooves first appear 

 be traced backward a differentiation in this area can 

 be found in a very early stage. We are thus enabled to 

 speak of an active pole and an opposite inactive pole. A 

 line passing through the two is designated as the primary 

 ovic axis. 



That the active pole or hemisphere gives rise to the 

 embryo was first pointed out by Jan. Swammerdam 

 in his 1 1 Bibel der Natur. ' ' This view was later supported 

 by Prevost and Dumas, von Baer, Reichert, Cramer, New- 

 port and others. Pftnger, however, believed that the 

 greater portion of the embryo was formed from the in- 

 active hemisphere and liis view was supported by Roux, 

 O. Hertwig and others. Most of the later investigators 



» With observations by C. O. Whitman on Bufo. 



504 



