2 34 REGENERA TION 



sooner and in proportionately larger numbers than do those from the 

 opposite hemisphere. The vegetative hemisphere would correspond 

 to that part of the egg from which the wall of the normal gastrula is 

 derived, and this may account for the clearer protoplasm of these 

 embryos, their inability in many cases to gastrulate, their larger cilia, 

 and the absence of mesenchyme in some of them. Driesch finds 

 that the number of cells that go into the mesenchyme of the partial 

 larvae is in proportion to the total number, and that the number of 

 cells in the archenteron is probably also proportionate. 1 



The smallest blastomeres that produce gastrulae are the one- 

 sixteenth products. Out of a total of 139 cases only 31 produced true 

 gastrulae, 5 produced gastrulae with evaginated archenteron, and 103 

 remained blastulae with long cilia. The one-thirty-second blasto- 

 meres were not observed to gastrulate. 



Driesch ('95) has also made a study of the potentialities of the 

 blastula and gastrula stages of sphaerechinus, echinus, and asterias. 

 If a blastula is cut in half before the mesenchyme cells are produced, 

 both pieces produce gastrulae and larvae. Since some of the pieces 

 probably come from the animal hemisphere, and others from the vege- 

 tative hemisphere, it follows that all parts of the blastula possess the 

 power of producing whole embryos, and in this respect the potentiali- 

 ties are the same as for the blastomeres. If the experiment is made 

 at a stage just before the archenteron has begun to develop (Fig. 65, 

 A), the results may be different. A half that contains the region 

 from which the archenteron is about to develop will produce a gastrula 

 and a larva (Fig. 65, A, lower row to right of A). A half that contains 

 only the opposite regions of the egg (Fig. 65, A, upper row) may in 

 some cases gastrulate, 2 often abnormally, but as many as half of the 

 pieces do not gastrulate. They may remain alive for a week or more, 

 and even produce a typical ciliated ring with a mouth in the centre, 

 but do not form a new archenteron. These important results show 

 that after the formation of the mesenchyme and archenteron at one 

 pole, the other cells of the blastula wall are no longer able to carry 

 out a process that the same cells were able to carry out at a slightly 

 younger stage, but whether this loss of power is connected with the 

 previous formation of the archenteron, or due to some other change 

 which has by this time taken place in the cells, cannot be determined 

 from the experiment. It is also important to note that these small 

 ectodermal blastulae can still develop whole, typical, ectodermal 

 organs, the ciliated ring and the mouth, and that the former especially 

 has the characteristic structure of the whole normal ring. 



1 Driesch's figures seem to show, nevertheless, that the archenterons are proportionately 

 too large. 



2 These may be pieces that were cut obliquely, as Driesch suggests, so that they con- 

 tain a part of the archenteric region. 



