gg K. MI'J'SUKURI ; 



the streak, e /, in the Woodcuts XII or XIII, aud removing the 

 exposed yolk-surface some distance backwards. 



How do chelonian embryos behave in this respect? The anterior 

 lip of the l)lastopore representing- here the posterior edge of the 

 blastoderm* is at first crescent-shaped with its concavity turned for- 

 wards (AYoodcut \ . 2). It then becomes gradually straight, and with 

 furtlier progress acquires a crescent-shape turned posteriorly (Woodcut 

 V. o, 4, 5, ()). It may possibly be pushing the comparison too far to 

 say that the median notch, noticeable in Woodcut V. o, 4, and 5, cor- 

 responds to the indentation in the median line of the posterior edge of 

 the blastoderm in Elasmobranchii (Sp:1)GWICK, '92 Fig. 1 = Sedwick, 

 Fig. 1, PI. VII of this article, or Balfour, Com. Embryol. Yol. ii, Fig. 

 30 A); but that such a notch is present in at least some chelonian em- 

 bryos, just at the stage when such ought theoretically to be visible, is 

 significant and ought not to be left out of sight. When the enclosure 

 of the volk-plug by the anterior lip of the blastopore has reached a 

 certain stage (Woodcut \ . 7, or Woodcut X, A p. 71) the lateral lips 

 of the blastopore press towards, and meet in, the median line, gradually 

 removing the yolk-plug or the exposed yolk-sui-face backwards and 

 producing the linear streak (the primitive streak) ])etween the original 

 position of the yolk-plug and its new one (Woodcut X, i?, C). 



In regard to the formation of the tail, there are some chrono- 

 logical disparities between Elasmobranchii and Chelonia. In the former 

 grouj), the foundation of the tail is laid before there is any streak 

 behind the embryo (Sedgwick, '92, Fig. 2 = Sedgwick, Fig. 2, PI. A"II. 

 of tlie present article). In the latter group, on the contrary, the 



* It may be urged by some that the sickle fçiven in some of the fio-iuvs of reptilian embryos, 

 as, for instance, in one copied from Kdpffer by Hertwig in Fig. 65 of his Lehrhuch (4te Aufl. 

 p. 95), represents the posterior edge of the lilastopore, from the analogy of Koller's sickle in 

 birds. But the sickle in the figure in question is formed by the accumumlation of the peristo- 

 mal mesoblast and has nothing to do with Koller's sickle. 



