CONCRESCENCE IN EMBRYO OF CRYPTOBRANCHUS. 257 



of the stained areas is apparent rather than real, since it is due 

 to the rotation of the entire egg on a horizontal axis. 



Figs. 36 to 43 require no explanation other than that given in 

 the legends. 



The large number of experiments performed to demonstrate 

 the movement of the neural folds gave remarkably uniform 

 results, and clearly indicate that the movement is one of transla- 

 tion, and not a mere wave movement or undulation. 



In distinction from the forms of concrescence more intimately 

 related to gastrulation, the mode of concrescence of the neural 

 folds and the gastral mesoderm might be distinguished as con- 

 crescence by apposition. 



IV. DISCUSSION. 



In this paper the term concrescence has been employed in a 

 wider sense than is usual in vertebrate embryology. One's views 

 as to the scope of the term concrescence will naturally depend on 

 his general theory concerning the origin of concrescence; on the 

 other hand that theory is the most acceptable that gives a valid 

 explanation of the largest number of facts, thus bringing them 

 into a common category. The theoretical interpretation that 

 appeals most strongly to the writer is the one stated by Wilson 

 ('90). As the volume containing this contribution is scarce and 

 almost inaccessible to many readers, a resume of the most 

 important points bearing on this problem is here given. 



There are two views as to the significance of concrescence. 

 According to the first view concrescence is a coenogenetic phase 

 of development; the embryo is temporarily bisected by the yolk, 

 and the two pajts afterwards brought together. Concrescence 

 is thus a process of restoration, by which the two halves of the 

 embryo, which have been mechanically separated by the yolk, 

 are united. According to the second view concrescence has a far 

 deeper meaning, and is palingenetic, though the accumulation 

 of yolk may have modified its character. The second view is 

 the one favored by Wilson, and requires further explanation. 



The origin of concrescent growth is to be sought in the origin 

 of bilaterality itself an inquiry which leads us to the problem 

 concerning the origin of bilateral animals from the ccelenterates, 



