236 REGENERA TION 



If the gastrula is cut in two in such a way that the inner end of the 

 archenteron, i.e. the part from which the pouches develop, is cut off 

 (Fig. 65, C\ it is found that the piece containing the posterior part 

 of the archenteron closes in, forms a new sphere, and from the present 

 inner end of the archenteron (that has also healed over) a pair of 

 pouches is produced (Fig. 65, C, lower row to right of C). These 

 pouches have arisen, therefore, from a more posterior part of the 

 archenteron than that from which the pouches normally arise. 



If the same experiment is made at a later stage, when the pouches 

 have been given off from the archenteron (Fig. 65, D, lower row to 

 right of D}, no new pouches are formed. This means that after the 

 archenteron has once produced its pouches it loses throughout all its 

 parts the power to repeat the process, although these parts possessed 

 this power at an earlier stage. It is a very plausible view that the 

 result is directly connected with the formation of the normal pouches, 

 although it is of course possible that some other change has taken 

 place in the archenteron that prevents the formation of the pouches. 



In order to give as nearly as possible a consecutive account of the 

 experiments on the eggs of the frog and of the sea-urchin, a number 

 of other discoveries have been passed over. Let us now examine 

 some of the results on other forms. 



Chabry, as early as 1887, experimented with the eggs of an ascid- 

 ian. By means of an ingenious instrument he was able to prick and 

 kill individual blastomeres. The results of his experiments were not 

 described very clearly, and later writers have interpreted his results in 

 different ways. 1 Chabry stated that he obtained half-embryos from 

 one of the first two blastomeres, but his figures show, especially in 

 the light of later work, that the embryos were whole embryos of half 

 size, although certain organs, as the papillae and the otolith, may be 

 lacking. 



Driesch ('95) reexamined the development of isolated blastomeres 

 in one of the ascidians, Phallusia mammalata, and found that the 

 cleavage of blastomeres, isolated by shaking, is neither like that of 

 the whole egg, nor is it like that of half the normal cleavage, although 

 it shows some characteristics of the latter. A symmetrical gastrula 

 is produced, and from this a typical whole larva of half size. These 

 larvae lack, however, one or more papillae, and the otolith rarely 

 develops. The absence of these organs Driesch ascribes to the 

 rough treatment that the egg has received, since embryos from whole 

 eggs may sometimes lack these organs if the development has taken 

 place under unfavorable conditions. The isolated one-fourth blasto- 

 mere may also produce a whole larva. 



Crampton ('97) has also studied the development of the isolated 



1 Driesch, Hertwig, Roux, Weismann, Barfurth. For review see Driesch ('95). 



