IQ (3 K. MITSUKURI; 



(lerm. Woodcut XVII, A shows two of tliese edges ap|)ro;iching efich 

 other. Either of tliese edges will hardly answer the description of a 

 growing edge by Hertwig, and wlien these meet (Woodcut XVII, B) 

 the line of junction has all the characteristics of a primitive streak. So 

 that there can be no question that the blastoderm edge of the Elasmo- 

 branch at earlier stages inay certainly be considered the blastopore 

 lips (Urmundrand). When the blastoderm of the Elasmobranch has 

 surrounded the yolk, as in Woodcuts XII or XIII (/). 01), what 

 structure the meeting edges, ef, and the edge round the yolk, r/, have 

 is a question which unfortunately I can not decide Iw appeal to actual 

 sections or to ßgures given by authors, for I do not possess the former 

 and I do not know of the existence of the latter. The nearest ap- 

 proach to it is in Figs. 2-1, I of Ziegler and Fig. 16, of Schwarz. 

 Both represent that region ventral to the tail where the blastoderm 

 edges are to ineet in a linear streak, but in the figures, there is still 

 the yolk exposed between the two edges. As far as I understand 

 these figures, the epiblast is reflected and becomes continuous wtih the 

 hypoblast, but the proliferation of the mesoblast is already given up. 

 I believe, the latter f ict may be explained by the consideration that the 

 blastoderm has been so long in encircling the yolk that the mesoblast 

 formation has been completed, and the middle layer has lost its 

 connection with the primary layei's. The blastoderm edge may there- 

 fore still be considered morphologically as the blastopore lips. At 

 any rate, it seems to me to have more the characters of blastopore 

 edge than those of a simple growing edge, especially when we remem- 

 ber what they were in earlier stages. Moreover, the parts in Chelonia, 

 which I trust to have proved to be homologous with the blastoderm 

 edg'e of Elasmobranchii, have unmistakablv the characters of the 

 blastopore lips. 



In regard to the edge of the blastoderm in Sauropsida, all that 



