Vcl. XXVI. May, r 9 i4. No. 



BIOLOGICAL BULLETIN 



AN EXPERIMENTAL STUDY OF CONCRESCENCE IN 



THE EMBRYO OF CRYPTOBRANCHUS 



ALLEGHENIENSIS. 1 



BERTRAM G. SMITH. 



I. INTRODUCTION. 



The concrescence theory is an attempt to establish a universal 

 law for the formation of the embryo. It is also an attempt to 

 apply to conditions found in the vertebrates, a law having a 

 wide range of validity in the invertebrates. 



In its widest sense, concrescence may be denned as the building- 

 up of the body of the embryo by the union along the median 

 line, of parts that are previously laid down as separate bilateral 

 foundations. The classical case is that of the leech (Whitman, 

 '78), in which the entire body, excepting the head, is formed by 

 concrescence. 



In the vertebrates the formation of the nerve tube by the appo- 

 sition of the neural folds, and the formation of the gastral meso- 

 derm in two more or less distinct halves which later unite across the 

 median line, come within the category of concrescence in this wide 

 sense of the term. But the problem has usually been restricted 

 to certain aspects of development intimately related to gastrula- 

 tion. Thus, in the teleost the convergence 2 of the germ ring 

 until its materials coalesce to form the posterior part of the em- 

 bryo, has been regarded as a process of concrescence. A very 

 similar process occurs, though less conspicuously, in the am- 

 phibian embryo. 



1 An abstract of this paper was read before the American Society of Zoologists 

 at the meeting in Philadelphia, December 2g, 1913. 



2 Since among authors there is no uniformity in the use of the words convergence, 

 confluence and apposition in their relation to concrescence, I have felt free to use 

 each term in the sense that seems to me the most appropriate. 



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