REGENERATION IN EGG AND EMBRYO 24! 



develop. Ziegler found that the micromeres still arise, and that from 

 such pieces larvae develop that have eight rows of paddles and four 

 endodermal sacs. In one case two of the sacs were smaller than the 

 others ; in another case one of the four was very much smaller than the 

 rest. In another operation a large piece was cut from the egg, leaving 

 a small nucleated piece that divided into two blastomeres of unequal 

 size. An embryo developed from this small piece with four endoder- 

 mal sacs, and only four well-developed rows of paddles. The four 

 rows of paddles that were lacking were represented by two groups of 

 a few plates each. 



Ziegler gives a different interpretation of these results from that 

 which Driesch and Morgan have offered. He interprets the last ex- 

 periment, in which after the operation the piece divided into two 

 unequal parts, and only four rows of paddles appeared, as meaning 

 that the development of these organs on the smaller part is sup- 

 pressed on account of the small size of the part. If the part had 

 been still smaller all trace of the missing paddles might disappear, as 

 he thinks was the case in certain experiments of Driesch and Morgan. 

 There can be, I think, little doubt that if a piece is small enough, the 

 result would follow as Ziegler supposes. It does not seem probable, 

 however, that the pieces were really below the lower limit in the 

 experiments of Driesch and Morgan, since the smaller blastomere 

 was in one case as large as the whole piece (i.e. as both blastomeres 

 taken together) in one of Ziegler's experiments. 



Ziegler's results show very clearly that we are not obliged to 

 think of the substance of the micromeres as laid down in the proto- 

 plasm of the egg, and hence there is no ground for supposing the 

 substance of the paddles is necessarily present in the vegetative 

 hemisphere of the egg. His results show that if the vegetative part 

 is cut off, micromeres and paddles are still formed, although that 

 part of the egg substance from which they normally arise has been 

 removed. It should be pointed out, in this connection, that Driesch 

 and Morgan did not suppose that the bases of the micromeres, or of 

 the paddles, are actually laid down in a definite part of the proto- 

 plasm of the egg. They had also observed that in some cases whole 

 embryos arose after a part of the egg had been removed, and this 

 they attributed to the symmetrical position of the cut in relation 

 to the organization of the egg. Ziegler's operations were made more 

 or less in this symmetrical plane, excepting the one that gave rise to 

 an incomplete embryo. Driesch and Morgan held that the formative 

 factors become localized in the protoplasm, rather than arise from 

 the nucleus, but pointed out that these observations do not lead to 

 His's conclusion of localized germ areas in the egg. 



