120 
inspired HIS to his theory, yielded only negative results. 
MORGAN in cutting the germ-ring at one side near the rudi- 
ment of the embryo yet obtained a perfect embryo. Accord- 
ing to him the embryo is “formed largely from material that 
has never been at the edge of the blastoderm” (p. 440). “At 
neither period is there sufficient material in the ring to 
form the sides of ihe embryo” (p. 463). 
SUMNER (1904) pricked with a needle the yolk just under 
the germ-ring, close behind the embryonic rudiment. After 
His’s theory this could be no hindrance to the further 
concrescence of the lateral borders beneath the puncture 
and we should expect to find the needle at the closure 
of the blastopore somewhere in the middle of the embryonic 
rudiment. This is not the case: the puncture is still found 
at the posterior end of the embryo, it has been pushed 
backwards by the overgrowing germ-disc. 
Similar experiments led KOPSCH (1896, p. 117) to the 
same conclusion. The caudal swelling (Endknospe) is the 
growing centre from which the trunk of the embryo IS 
formed and which indicates its hinder end. Only a very 
restricted part of the germ ring contributes towards the 
formation of the embryo. The head, however, is not formed 
in this way, it originates in loco (p. 121). 
The same conclusion is drawn by KATSCHENKO (1888, 
p. 456) from similar experiments on Selachians. 
Spina bifida. — From a study of pathological forms HERTWIG 
(1892) likewise concludes that the embryo is formed by 
concrescence. Eggs developing under somewhat abnormal 
conditions often show the phenomenon first discribed by 
ROUX (1888) as Asyntaxia medullaris and afterwards studied 
by HERTWIG under the name Spina bifida. It is characterized 
by the peculiarity that the contraction of the blastopore border 
is retarded and the differentiation of the medullary plate, the 
mesoderm and the notochord begins while the blastopore 
is still wide open. The rudiments of these organs then 
appear to surround the blastopore and the yolk-cell mass 
as a ring. From this HERTWIG concluded that the whole 
embryo is formed in normal development by concrescence 
of the lateral borders of the blastopore, the transverse 
brainfold lying close in front of the place where the dorsal 
blastopore rim first appears, as had been asserted by 
ROUX (cf. p. 128). Since the rudiment of the embryo of 
the frog has a length of some 180° of the circumference 
