11^ K. MITSUKURI; 



failed to notice either the posterior elomjation of the primitice streak and 

 groove or the hachvard moceinent of the yolk-plug. It would be rash in 

 7ne to say how the course of events shapes itself in Lacerta in regard 

 to these points, but that groove behind the medtilJary canal seen in 

 Will's Figs. 22, 23, 24, 25, 2ßa, and 27a could only have been 

 produced by a process similar in nature to that which I have described 

 in the foregoing article for the three species of Chelonia. This 

 conviction is all the more strengthened, because iu his Fig. 25 a struct- 

 ure which strongly reminds one of such a yolk-plug as I have described 

 in the foregoing article is visible at the posterior end of the pr-imitive 

 groove. Also in his Fig. 21, the posteri(3r ends of the medullary 

 folds embrace between themselves a wedge-shaped structure which can 

 be nothing else than the yolk-plug. Will himself explains this by 

 saying that the anterior halves of the primitive plate have come 

 t(jgether faster than the posterior halves. I believe that if the posterior 

 end of the primitive streak in such Lacerta embryos as are represented in 

 Will's Figs 22-2 7ci is critically examined, there will be found a yolk- 

 plug, which, if not visible in surface-views, ought to be recognisable in 

 sections. As I have said in the foregoing article, the yolk-plug at such 

 stages is very difficult to detect. In some Triongx embryos, I could 

 detect it only by turning theui round and round in the direct sunlight 

 or on clear bright days, when the light was reflected at a certain 

 angle. It is possible that if I had not had comparatively clear pictures 

 of Chelonia and Glemimjs for comparison, I should never have 

 suspected the existence of the yolk-plug in Triomjx which would thus 

 have remained unknown to me. It seems to me that this consideration 

 accounts for Will's failure to recognise the yolk-plug. As the posterior 

 elongation of the primitive groove is, however, well marked in Lacerta^ 

 as his figures show, the yolk-plug may perhaps be recognisable with 

 comparative case, if once one looked for it. What 1 am inclined 



