REPRODUCTION AND DEVELOPMENT 



287 



round mass of cells. Now if the alcohol is removed before the injury 

 is fatal, new mouths equipped with tentacles appear at several points 

 around the edge of the flattened mass. It is interpreted to mean that 

 the depressant action of the alcohol has extinguished the relation of 

 dominance and subordination that obtains between the apical or 

 mouth region and the remainder, and that new dominant regions 

 arise when this depression is removed. 



One of the most common human congenital defects is harelip. 

 It results from the failure, or partial failure, to fuse on the part of 



INDUCED 

 NEURAL 

 TUBE 



NORMAL 



NEURAL 



TUBE 



Fig. 193. — An early salamander embryo with a normal neural tube and one in- 

 duced to form by a small portion of the lip of the blastopore of another embryo 

 transplanted into the side region (After Spemann and Mangold.) 



the two processes that grow out. from the primordium of the lower 

 jaw on either side. Normally these two maxillary processes meet 

 at the mid-line and fuse to form the upper jaw, upper lip, and 

 palate. Harelip has no significance whatever, except the incomplete 

 fusion of these two processes; it is not possible to state exactly the 

 cause of this growth deficiency. 



While much of the detail of the nature of the processes that regu- 

 late normal form in the development of the individual, and of their 

 upset by environmental conditions, is yet obscure, in a general way 

 abnormalities can be accounted for in terms of abnormalities in 



