FATE OF BLASTOPORE, ETC., IX CHELOXIA. 55 



Fig. 9 is the stage in which the medullary folds have reached pos- 

 teriorly the yolk-plug and compress it laterally. The medul- 

 lary canal is still open throughout. At the posterior cephalic region 

 the medullary folds, which first arose here, are now pressed ;igainst 

 each other, thus narrowing the medullary canal in that reoion in 

 the surface view. 



Fig. 10 is a sketch of an embryo, made long ao-o and 

 produced in Fig. 2 of Contrih. II. I considered at the time that the 

 horse-shoe shaped yolk-plug marked the posterior end of the medullary 

 canal. A study of this figure, along with the series of sections prepar- 

 ed from the embryo represented in it, has made it evident that this is 

 the stage in which the yolk-plug has already retreated a short distance 

 behind its initial position. A slight enlargement of the medullary 

 fissure a short distance in front of the yolk-plug, which I noted and 

 sketched at the time, is undoubtedly the posterior end of the medullary 

 canal and the line between that enlargement and the yolk-plug 

 represents the primitive groove. This will l)ecome clear from the 

 description of the sections prepared from the embryo (Series III. Pi. 



IV.). 



Fig. 11 is the stage corresponding to Fig. 5 or 6 oî Clidonia. 

 The amnion leaves only the posterior part of the embryo exposed. 

 As in Chelonia, the posterior end of the medullary folds is seen 

 merging in the " Endwulst," which stands out as a circular white 

 area. The medullary canal is completely closed, with the exception 

 of a little stretch at its posterior end. At this place, there is a 

 longitudinal opening, easily distinguishable by being wavy and 

 a little wider than the rest of the median streak. This opening lies 

 in a surface view at the anterior median radius of the " Endwulst." 

 This also marks the position of the neurenteric canal in a surface view. 

 From this opening, a delicate, and narrow groove — the primitive 



