DEVELOPMENT OF SPELERPES BILINEATUS 193 



blastopore, plate 1, figs. 7 and 8. At its point of origin, it is 

 slightly deeper than in the remaining parts. 



Plate 1, figs. 9 and 10. The neural region is slightly elevated 

 anteriorly and is distinctly differentiated throughout its entire 

 extent. At the anterior border, the neural folds are not yet 

 raised above the general surface of the egg, although they can be 

 made out without much difficulty. In the lateral regions, they 

 begin to ascend gradually as two distinct ridges, now consider- 

 ably broader than before, reaching their highest point anterior 

 to the middle of the neural area. Posterior to this point, they 

 gradual^ descend until only faintly discernible. In this particu- 

 lar egg, the}^ appear to surround the blastopore. In other eggs, 

 this fact is not evident. The usual further development of the 

 folds is well shown by figs. 11-14, pldte 1, without comment. 

 Sometimes the folds remain parallel while closing. 



When the neural folds are nearly closed (fig. 13, plate 1), a 

 slow rotation of the embryo about its horizontal, longitudinal 

 axis begins, so that in the course of a few hours the embryo lies 

 on its side and often on its back. Later it rotates constantly 

 about a vertical or oblique axis, sometimes slowly, sometimes 

 rapidly. It may rotate head or tail foremost. This rotation 

 is presumably due to cilia, although I have never seen them. 

 Such rotation of amphibian embryos has been noted by Clark 

 ('80), Eycleshymer ('95), Assheton ('96) and Wilson ('97). 



From the time the neural folds become well-defined throughout 

 their entire extent, the embryo rapidly elongates, so that when the 

 folds have closed, it extends 'over 315°, being curled ventrally 

 about the yolk cells. The head and tail are thus brought close 

 together. While the question of elongation has not been care- 

 fully studied, there is some evidence to show that the gre.iter 

 part of this elongation takes place in the posterior part of the 

 animal. In several cases among the stained eggs to be described 

 later, marks made in the posterior regions have lengthened. 

 In the anterior regions, on the contrary, such elongation has not 

 been noted. 



These marked eggs show also that the neural folds close by an 

 actual transference of the neural plate material and not by a 



