REGENERATION IN EGG AND EMBRYO 231 



Wilson ('93) studied the development of isolated blastomeres of 

 amphioxus, and found that it agreed in all essential respects with 

 the mode of development of the blastomeres of the sea-urchin. The 

 isolated blastomeres of the two-cell and four-cell stages produce whole 

 embryos, but the blastomeres of the eight-cell stage develop only 

 as far as the blastula. The blastomeres segment, after separation, 

 in most cases not as a part, but as a whole egg would divide, 

 although the cleavage of the one-eighth blastomere only approaches 

 that of the entire egg, but is never identical with it. Incompletely 

 separated blastomeres give rise to twins, triplets, etc. Wilson agreed 

 with the Hertwig-Driesch conception of the value of the early blasto- 

 meres, and accepted the view that the fate of each is a function of 

 its position, and that at first they are qualitatively alike. During the 

 early cleavage he supposed that a change takes place that is slight 

 at the two-cell stage, greater at the four-cell stage, and in the eight- 

 cell stage the differentiation has gone so far that the blastomere can 

 no longer return to the condition of the ovum. "The ontogeny 

 assumes more and more the character of a mosaic work as it goes 

 forward." 



Loeb ('94) showed that if the eggs of the sea-urchin are placed in 

 sea water, diluted by distilled water, the egg swells and bursts its 

 membrane, so that a part of its protoplasm protrudes. Into this pro- 

 trusion some of the first-formed nuclei pass, and from both the part 

 remaining in the egg membrane, as well as from the protruding part, 

 an embryo is produced, the two embryos often sticking together. In 

 several cases two to eight separate groups of blastomeres are formed 

 from one egg and develop into whole embryos. 1 



The question of the number of cells which are produced by the one- 

 half and one-fourth embryos had not up to this time been determined. 

 Until this was known it could not be stated whether the smaller 

 embryos were miniature copies of the normal embryos in all respects, 

 or whether they assumed the typical form with fewer cells. I found 

 ('95) that the blastula from one of the first two blastomeres contains 

 half the number of cells produced by the whole embryo, and that in 

 the later stages also it contains only about half the normal number. 

 The one-fourth blastomere produces only a fourth of the whole num- 

 ber of cells, and yet can develop with this number, in many cases, 

 into a whole embryo. The one-eighth blastomere produces one- 

 eighth the normal number of cells. In most cases I found that these 

 one-eighth blastomeres do not produce embryos, but occasionally 

 they produce a gastrula, and probably a young pluteus stage. 



1 The evidence to show that more than four and certainly more than eight such groups 

 that come from a single egg can produce a pluteus is, I think, insufficient, and the result 

 improbable. 



