DOUBLE MONSTERS IN FISHES 57 



workers that the germ ring does not concresce to form 

 the embryonic axis. 



kopsch's view of embryo formation 



In a very able monograph entitled Untersuchungen 

 uber Gastrulation und Emhryo-hildungen hei den Chordaten, 

 Kopsch (1904) has brought out the following facts: 



a) If with a hot needle a portion of the germ ring 

 near an early embryonic shield is killed, such injury has 

 no effect upon the organization of the future embryo 

 but merely results in the formation of a scar in the tissue 

 outside of the embryo proper. This was true no matter 

 how close to the embryonic shield the injury was made. 



b) If the posterior end of the embryonic shield is 

 injured, the entire embryonic body is formed except 

 the tail and adjacent tissues. This means that the 

 whole body is represented in the original embryonic 

 shield and is not built up by germ-ring concrescence. 



c) The embryonic shield, as soon as it is definitely 

 established, contains the complete embryonic axis from 

 head to tail. At first the tail proper is represented only 

 by a tail-bud, which begins to grow out even before the 

 germ ring has completed its envelopment of the yolk. 



d) The only evidence of any concrescence at all is 

 seen in connection with the formation of the Knopf or 

 tail-bud region. At first this region appears to arise 

 at the two outer margins of the thickened portion 

 destined to form the embryonic shield. These bilateral 

 regions appear to grow in, apparently more by migration 

 of cells than by gross concrescence, to form a median 

 posterior area of the shield which is essentially the 

 tail-bud. 



