232 RE GENERA TION 



The development of nucleated fragments of the egg was also 

 studied in order to find out if they too produce a smaller number of 

 cells than does the whole egg, and a number in proportion to their 

 size. The problem is different in this case, because the nucleus has 

 not divided before the piece is separated, and the results ought to 

 show whether there is a prescribed number of divisions for the egg 

 nucleus, or whether the number of times it divides is regulated by 

 the amount of the protoplasm. It was found that the number of 

 cells produced by each fragment is in proportion to the size of the 

 piece. This shows that the division of the nucleus is brought to an 

 end when the protoplasm has become subdivided to a certain point. 



A further examination of the number of cells that are invaginated 

 in these smaller " partial " larvae to produce the archenteron seemed 

 to show that they often use relatively more than their proportionate 

 number. The normal blastula of Sphcerechinus grannlaris contains 

 about five hundred cells and turns in fifty cells, or one-tenth the total 

 number. The one-half and one-fourth embryos, and some of the 

 small embryos from the egg fragments, seemed to invaginate more 

 than one-tenth of their total number of cells. 



Driesch (1900) reexamined this point, and found that the em- 

 bryos from isolated blastomeres may use the proportionate number 

 of cells. I have made a new study of the problem on a larger scale 

 and have found that my earlier statement, as well as that of Driesch, 

 is substantially correct, and that the difference that we found is due 

 to the time at which the embryos gastrulate. Thus the one-half 

 embryos and even the one-fourth embryos, that gastrulate as soon as 

 (or only a little later than) the normal, whole embryos, turn into the 

 archenteron about one-half and one-fourth the number of cells 

 invaginated in the whole embryo ; but those partial embryos that 

 gastrulate later (as most of them do) turn into the archenteron more 

 than a half or a fourth of the number of cells turned in at first 

 by the whole embryo. This difference between the early and the 

 retarded partial embryos is in large part due to a slow increase of 

 cells that takes place during the delay in development. 



Driesch ('95) found that pieces of the blastula wall of the sea- 

 urchin, if large enough, can also produce a gastrula and embryo. I 

 found that the number of cells in these pieces does not increase 

 appreciably after they are cut off (if the operation has been carried 

 out at the end of the cleavage period), and that the new embryo is 

 organized out of the cells present at the time of removal of the piece 

 from the wall. There is, therefore, in this case no chance for " post- 

 generation " by means of new cells produced at the side, which Roux 

 has supposed to take place in the frog embryo. 



The development of pieces of the blastula wall, if they are not too 



