26O BERTRAM G. SMITH. 



Like most phylogenetic speculations, the palingenetic theory 

 of concrescence is weak in that it seems incapable of absolute 

 demonstration. On the other hand the theory is useful in that 

 it brings together in a single view a number of very important 

 anatomical and embryological facts, putting them in an intel- 

 ligible relation to one another. If we reject this theory, then we 

 can scarcely consider the term concrescence to cover the be- 

 havior of the neural folds and the mesoderm in the vertebrates; 

 concrescence becomes limited to events concerned with the 

 present process of gastrulation, and probably applies only to the 

 formation of the posterior end of the embryo. If we accept this 

 theory, then concrescence has a broader and deeper meaning, 

 and explains the most fundamental events in the formation of the 

 embryo. 



Perhaps the greatest gain that has come from the theory or 

 theories of concrescence is that research has thereby been stimu- 

 lated, with the result that we now have a much more accurate 

 knowledge of the early stages of development of many forms 

 than would otherwise be the case. The facts thus brought to 

 light may be of greater value when considered from an altogether 

 different angle, in the development of the various aspects of 

 embryological science. 



V. SUMMARY. 



By the method of vital staining the following facts concerning 

 the formation of the embryo of Cryptobranchus were established : 

 (i) A band of cells occupying the lateral and ventral parts of the 

 equatorial region of the late blastula, during gastrulation comes 

 to occupy the corresponding parts of the margin of the yolk plug, 

 and converges on the site of the closing blastopore. (2) During 

 gastrulation there is a confluence of material lying in the region 

 of the dorsal lip of the blastopore : in connection with the process 

 of overgrowth and in-turning of the dorsal lip of the blastopore, 

 this material shifts from either side toward the median line. (3) 

 The movement of the neural folds is a movement of translation, 

 not a mere wave movement; the neural folds include material 

 originally situated at least 90 apart, which is thus brought into 

 apposition in the median line. 



