136 Organisms from Eggs 



hollow sphere; under normal conditions, the resistance 

 on the inner surface being smaller, the intestine grows 

 into the hollow sphere. 



The intestine is one of the organs required for the 

 self-preservation of a more complicated organism, in 

 fact a higher organism without a digestive tract is not 

 capable of living for any length of time. In the gastrula 

 — i. e., the blastula with an intestine — we have an 

 organism which is durable, but the processes leading up 

 to the formation of the intestine are so simple that it 

 is difficult to understand why the assumption of a 

 *'supergene" should be required in this case. 



3. Driesch^ was the first to show that if we isolate 

 one of the first two cells of a dividing egg each develops 

 into a whole embryo of half size. This is perfectly 

 intelligible, since each of the two cells contains all the 

 three layers in the normal arrangement (Fig. 10). The 

 cells divide and the cells having the tendency to creep 

 to the surface of the mass arrange themselves in a hollow 

 sphere, the blastula. Since micromeres and intestine 

 material are present and in their normal position an 

 intestine will grow into the blastula and a whole or- 

 ganism will result. All of this is as necessary as is the 

 formation of one embryo from the whole egg material. 

 Yet the two half-embryos betray their origin from tw^o 

 cleavage cells of the same egg, in that the two gastrulae 

 formed are often if not always symmetrical to each 



» Driesch, H., Ztschr. f. wissnsch. ZooL, 1891, liii., 160. 



