178 



THE ANCESTRY OF VERTEBRATES 



(1895) and after them by KOPSCH (1900), according to 

 whom the egg axis lies in the embryo from a ventral poini 

 in front to a dorsal point behind. If SCHULTZE was of 

 opinion that the formative material of the embiyo lies 

 entirely in front of the dorsal blastopore border, and if ROUX, 

 Hertwig, Bertacchini that at first it surrounds the blasto- 

 pore as a ring, then according to KOPSCH there is some truth 

 in both statements, the rudiment of the head being found 

 in front of the newly formed blastopore lip, the contiguous 

 rudiment of the dorsal parts of the trunk lying round the 



Fig. 34. Situation of the medullary plate in the frog egg according to 

 SCHULTZE (a) and Roux (b). 



after Roux, 1888, p. 698. 



blastopore border in the semilunar stage of the latter. This 

 last view is more and more accepted by later investigators 

 (H. V. WILSON, 1900, 1902, KING, 1902, IKEDA, 1902) and 

 also my experiments confirm it entirely, as will be shown. 

 The view is gaining ground that the principal axis of the 

 egg and the longitudinal axis of the embryo more or less 

 coincide and that consequently, when the first cleavage of 

 the frog egg separates the left and right halves of the 

 embryo (which is so in the majority of cqses, see note on 

 p. 181), the second cleavage will not separate rostral and 

 caudal but dorsal and ventral parts of the embryo. Meanwhile 

 opinions still differ widely; thus BRACHET ;(1902. 1905) 

 has recently supported the view that the transverse head 

 fold originates exactly in front of the spot where the 



