GASTRULATION A^'D EARLIEST DEVELOPMENT 169 



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 the embryo" (p. 463). 



Sumner (1904) pricked with a needle the yolk just under 

 the germ-ring, close behind the embryonic rudiment. After 

 HlS'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) concludes that the embryo is formed by 

 concrescence. Eggs developing under somewhat abnormal 

 conditions often show the phenomenon first discribe'd 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. 177). Since the rudiment of the embryo of 

 the frog has a length of some 180<* of the circumference 



