129 
(1895) and after them by KOPSCH (1900), according to 
whom the egg axis lies in the embryo from a ventral point 
in front to a dorsal point behind. If SCHULTZE was of 
opinion that the formative material of the embryo 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 cases, see note on 
p. 132), 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 
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