226 REGENERATION 



whole embryo develops, as in Schultze's experiment. In one case I 

 obtained a half-embryo from an inverted egg. The result did not 

 appear to be due to a lack of rotation of the protoplasm, because the 

 medullary folds were white, showing that the protoplasm must have 

 changed its position. The result can possibly be explained as due 

 to the protoplasm rotating in each blastomere along the line between 

 the halves, so that it still retains the same relation as that of the 

 normal two-celled stage. 



The whole embryos of half size are generally imperfect in certain 

 respects on account of their union with the other half. They resemble 

 in all important points the embryos described by Hertwig, and I see 

 no grounds for interpreting them as embryos of a meroblastic type, 

 but rather as whole embryos of half size, whose development pos- 

 teriorly and ventrally has been delayed or interfered with by the 

 presence of the other blastomere. 



It has not been possible to separate the first two blastomeres of 

 the frog's egg, for if one is removed the other collapses. In the sala- 

 mander, that has a mode of development similar to that of the frog, 1 

 it has been possible to separate the first two blastomeres. Herlitzka, 

 who carried out this experiment, found that each blastomere gives rise 

 to a perfect, whole embryo of half size. We cannot doubt, I think, 

 that the same power of producing a whole embryo is also present in 

 each of the first two blastomeres of the frog's egg. When the two 

 remain in contact in their normal relation to each other, each produces 

 only a half ; when like regions of the two blastomeres are separated, 

 each produces a whole embryo. Thus we see that whatever the fac- 

 tors may be that determine the development of a single embryo from 

 the egg, still each half, and perhaps each fourth also, has the power of 

 producing a whole embryo. 



In later papers Roux has stated that he had also, even in his 

 earlier experiments, found other kinds of embryos than the half-em- 

 bryos that he described. Some of these were whole embryos that 

 had developed from the uninjured blastomere without the injured one 

 taking any part or only a very small share in their formation. He 

 found, he states, all stages between those embryos that had used up 

 all the yolk material of the injured side (though post-generated) and 

 those that had not used any part of it. The latter kind of embryo he 

 does not recognize as a whole embryo of half size in the sense that a 

 single blastomere has developed directly into a smaller whole embryo, 

 but he believes that there must have been formed at first a half-blas- 

 tula, half-gastrula, half-embryo, and that the last stage completed 

 itself laterally without using any material from the injured half. 



1 The plane of the first cleavage has been shown in two urodeles to correspond, not to the 

 median longitudinal plane, but to a cross-plane of the embryo. 



