50 LIBBIE H. HYMAN. 



cells around the shield probably take part. Thus Morgan ('95) 

 found that in Tautogolabrus the embryo is formed by a concen- 

 tration of material towards the center of the shield. Sumner 

 ('03) states that if the embryonic shield of Fundulus is destroyed 

 a new embryonic shield is regenerated. My observations also 

 show that in Tautogolabrus and the cod the region about the 

 shield takes part in the formation of the embryo since a large area 

 of high susceptibility is present in the early stages, as seen in my 

 Figs. 18 and 20 to 26. While materials around the shield may 

 contribute to the embryo the work of Kopsch, Morgan, and 

 Sumner agrees in denying such a role to the germ ring, in the 

 formation of the early embryonic axis. Cuts or injuries in the 

 germ ring at the sides of the shield do not affect the formation of 

 the embryo but this proceeds in practically normal manner. 



Although according to these lines of evidence the germ ring 

 plays no role or only a very minor one in the early development of 

 the embryo, it sooner or later becomes involved in the formation 

 of the embryo. This is shown in all three species by the appear- 

 ance of a region of high susceptibility at the posterior end of the 

 embryonic axis. This active region arises very early in Fundulus 

 and this fact indicates that the greater part of the Fundulus 

 embryo is laid down by this posterior growing region. This was 

 also the conclusion reached by Sumner since he found that pierc- 

 ing this growing region inhibits the formation of the posterior 

 part of the embryo. This region, grows backwards adding to the 

 embryo in front of it, exactly as in the case of the primitive 

 streak of the chick, with which it is no doubt homologous. In the 

 cod the posterior growing region arises somewhat later; and in 

 Tautogolabrus very late. In the latter species it is evident that 

 very little of the axis of the embryo is due to the activity of this 

 posterior region and that the embryo elongates considerably with- 

 out it, apparently through growth at its anterior end. This inde- 

 pendence of the embryo of Tautogolabrus of the germ ring was 

 already noted by Morgan in 1895 since he found that by placing 

 the eggs in diluted sea-water it was possible to retard or prevent 

 the formation of the embryo without affecting the growth or 

 closure of the germ ring. Experiments on other species are 



