No. 2.] FORMATION OF THE FISH EMBRYO. 44 1 



right posterior end of the embryo is completely cut off from 

 the germ-ring of that side. A cleft in the extra-embryonic 

 area runs out from the blastopore. It will be seen that the 

 germ-ring, cut off from the embryo on one side, has never again 

 reunited, but nevertheless a perfect embryo has formed, as 

 drawn in C. 



Cross-sections through this embryo show that the mesoderm 

 on the injured side of the embryo, in the region of the tail- 

 knob, is almost absent. Farther forward there is less meso- 

 derm on the injured side than on the normal side, but in the 

 middle and anterior part of the embryo the same quantity of 

 mesoderm is present on both sides. 



At the posterior end of the body the ectoderm has not 

 turned in so perfectly to form the central nervous system, and 

 it is due to the presence of lateral ectoderm that, in the 

 absence of the proper amount of mesoderm, the external sym- 

 metry seems to be perfectly preserved. 



The result of another experiment is shown in Figs. 40, A, B. 

 Here the germ-ring was cut at the side of the embryo before 

 the complete establishment of the head. The embryo was 

 killed at a stage when it had still a large blastopore {B). To 

 the left of this embryo a line of dead cells (or abnormal cells) 

 marks the line where the earlier severance of the germ-ring 

 took place. It will be seen that an imperfect union has taken 

 place between the germ-ring and the embryo along the line of 

 necrotic cells. With this exception, the embryo is normal. 

 To the outer side of these necrotic cells there is a thickened 

 mass of cells continuous into the germ-ring. 



I should interpret this result to mean that after the sever- 

 ance of the germ-ring the head established itself, and the 

 posterior extension of the embryo followed. Owing to the im- 

 perfect reunion of the germ-ring with the embryo, an accumula- 

 tion of cells has taken place on the outer edge of this barrier. 

 It looks as though this mass of cells belonged to the embryo, 

 but was prevented from passing into it by the hindrance due 

 to the wound. If this interpretation be correct, two conclu- 

 sions follow that are of importance in forming any conception 

 of the relation of the germ-ring and embryo in the Teleost : 



