440 MORGAN. [Vol. X. 



first non-nucleated, — the fact that this subsequently becomes 

 nucleated and goes into the mesoderm at the side of the 

 posterior end of the body, is of special interest, and needs to be 

 especially emphasized. It is probably in part due to this that 

 an embryo larger than half is formed from one of the first two 

 blastomeres when the first two blastomeres are of equal size. 



Cutting the Germ-ring. 



In my preliminary communication I have described a series 

 of experiments, by means of which I believe that it has been 

 shown, experimentally, that as soon as the head is definitely 

 established it remains a fixed point, and the elongation of 

 the embryo takes place posteriorly. When the germ-ring first 

 appears it is thickened along one side. On this side the head 

 rapidly forms, reaching from the thickened germ-ring to the 

 apex of the early blastoderm, afid formed largely from material 

 that has never been at the edge of the blastoderm. 



In another set of experiments I attempted to cut the germ- 

 ring to one side of the embryo. ' In a few cases in which the 

 germ-ring was cut at one side of the embryo, with a hot needle, 

 the embryo continued to develop, although the germ-ring 

 remained intact only on the uninjured side. In several cases, 

 out of a very large number, where the germ-ring was cut in 

 two by a sharp cold needle, the cut ends drew apart and did 

 not unite again.' From these experiments I drew the conclu- 

 sion: The embryo, cut off from all connection with the germ- 

 ring on one side, elongates backwards, producing an embryo, 

 having both right and left sides alike and equal. In the elon- 

 gation of the embryonic knob backwards, the head remains a 

 fixed point, and the elongation is due to an extension back- 

 wards of the mass ; the germ-ring, therefore, takes no im- 

 portant part in the formation of the body of the fish-embryo. 



In support of these conclusions I now figure two of these 

 embryos. In PI. Ill, Figs. 39, A, B, C, we find the germ- 

 ring cut in A, at the time when the head has just been 

 established. The embryo was killed when the germ-ring had 

 not yet closed, as seen in B. We see in this figure that the 



