CONCRESCENCE. 



245 



taken place, apical growth can no longer proceed over the surface of the yolk, it 

 must take place by the piling up of one segment on top of another, in the shape 

 of small closed rings. 



Concrescence, therefore, is the inevitable result of apical and bilateral growth 

 over a spherical yolk surface. 



The amount of concrescence in a given animal depends on the ratio of 

 marginal to apical growth, and upon the radius of the yolk sphere. Another 

 factor that materially affects the form of the embryo, is the gradual suppression 

 of the lateral margins of the more anterior segments. 



Fig. 154. — ^Young Limulus, after the shedding of the trilobite shell. The segmental sense organs of the fourth 

 thoracic segment, and the procephalic sutures of the trilobite stage, have disappeared. 



Embryonic apical growth represents the phylogenetic method of adding new 

 body segments to the old, but it is accomplished in a very different manner in 

 one case from the other. The embryo, for example, produces a succession of flat 

 bands on a curved, or flat surface, each one arising under different conditions from 

 the preceding one, and forming closed rings as best they may. In the adult an- 

 cestral forms, all the new segments were produced under essentially like con- 

 ditions, that is, as small closed rings at the apex of a cyhnder. 



In Limulus, an oblong area is enclosed between the posterior part of the con- 

 crescing germ wall and the broad anal plate. This area and its surrounding 

 germ wall, I shall call the caudal navel. (Fig. 141, c.nv.) 



The inward proliferation in this region is very conspicuous and might be 



