58 THE ORIGIN OF THE NERVOUS SYSTEM 



Bilaterality is inherited through many processes of 

 agamic reproduction, e.g., in flatworms and annelids, 

 and may of course also be inherited in many eggs. On 

 the other hand, it may conceivably be determined in 

 some eggs by ovarian conditions, by conditions connected 

 with maturation or with fertilization or perhaps even by 

 conditions arising later in development. For example, 

 the median ventral trunk region in the polychaete 

 annelids arises from the fusion or concrescence of the 

 growing borders of the somatic plate (Child, 1917c?), 

 and in the vertebrates from the concrescence of the 

 growing border of the blastoderm or the lips of the 

 blastopore. The higher frequency of cell division and 

 the smaller size of cells in the lateral border of the 

 somatic plate of various annelids suggest that it is a 

 region of high physiological activity, and in certain 

 cases I have found that it is more susceptible to Jethal 

 concentrations than other parts of the somatic plate. 

 Hyman has observed that in certain of the fishes the 

 border of the blastoderm becomes a region of high 

 susceptibility as it begins to form the germ ring and to 

 grow down over the yolk (unpublished). As the embryo 

 is formed, this active border undergoes concrescence to 

 form the median dorsal region. In the frog (Bellamy, 

 1 9 19) after gastrulation the lips of the blastopore, which 

 are overgrowing the yolk, constitute a region of high 

 susceptibility to lethal concentrations, most marked in 

 the dorsal region. These lips form the median dorsal 

 region of the embryonic body. These facts suggest 

 that the mediolateral gradients in the annelids and in 

 at least the lower vertebrates may be associated to 

 some extent with the differences in physiological con- 



