Concrescence Theory 5 



general size, or in length only ; it may increase as a whole, or in 

 part ; in any case the increase comes from a general multiplication 

 of its cells, that is to say, it is the result of a general interstitial 

 growth. 



Or again there may be certain definite centres which continue 

 for some time as centres of indifferent cell proliferation, and which 

 give rise to growth in length of parts, as for instance, in the 

 case of the limbs, or of the body as a whole. 



Now in the particular case which we are considering, namely 

 the growth in length of the fish embryo, it might be possible 

 theoretically to ascribe the change from the condition seen in 

 the earlier stage A, Fig. 3, to that of the later stage B, Fig. 3, 

 as being due to one of three causes. 



B C 



Fig. 3. Concrescence theory, after His. 



The arrows indicate the direction in which the edges are approaching 

 each other ; the dotted area represents yolk. 



1. It might be due to a general interstitial growth whereby 

 a b becomes a'--b'. In this case the foundation of the whole 

 embryo would be considered as having been definitely laid down 

 between these two points from the beginning. 



This explanation however is probably not correct, because, 

 as is perfectly well known, in all vertebrate embryos the anterior 

 part of the embryo develops long before the hindermost. In Fig. 3 

 the earlier stage shows only the rudiment of the fore part of 

 the brain. The rest of the brain and the spinal cord and the 

 protovertebrae are added on subsequently, in regular succession. 



Experimental evidence is also against this hypothesis. 



2. Or, we may regard the advancing rim as being an area of 

 special cell proliferation, and as giving rise to the more posterior 

 part of the embryo as it passes backwards over the yolk. 



