MODERN BIOLOGY 581 



be explained by an example: the half-egg of the sea-urchin has the pro- 

 spective value of forming half a larva, but the potentiality to form a whole 

 larva; the former is thus the determination of the part that holds good in 

 normal cases, the latter the power of every part to compensate for another 

 if necessary. On the other hand, Roux found support in the American Wilson. 

 He isolated cleavage cells from the egg of a mollusc and found that they 

 became what they would have become in their normal connexion, and noth- 

 ing more. And later Boveri discovered that the sea-urchin's eggs investigated 

 by Driesch actually possess a differentiation that from the beginning deter- 

 mines the succeeding developmental orientation. The fact is, the disputants 

 have gradually had to reconcile their views; Roux had to abandon his theory 

 of the different-shaped cleavage of the nuclei, while Driesch had to give up 

 his theory of the absolutely uniform character of the sea-urchin's egg. On 

 the whole, these experimental discoveries were too generalized; what was 

 discovered in the case of one animal's egg was applied without question to 

 the entire animal kingdom, whereas in actual fact a vast number of different 

 conditions prevail. In the main, however, it seems as if subsequent research 

 has obtained results that would indicate generally that a very early special- 

 ization and localization of the rudiments take place in the embryo. The 

 investigations that have been carried out especially by Hans Spemann, one 

 of the foremost champions of experimental morphology at the present time, 

 give some evidence of this. Born in 1869 at Stuttgart, he studied at Heidel- 

 berg, Munich, and Wiirzburg, and has been a professor at Rostock and 

 at the Kaiser Wilhelm Institute in Berlin; he is now working at Freiburg. 

 An extremely clever experimenter, he has specially taken upon himself to 

 prove the determination of different sections of the embryo by transferring 

 parts of the body of a batrachian embryo from one place to another and 

 then studying the successive development. As a general result he records that 

 the main organs of the amphibian embryo are definitely determined during 

 the process of gastrula formation; at an earlier period, in the blastula stage, 

 normally formed twins can be produced by means of dividing off with thread; 

 at the beginning of the gastrula stage the forward end can still be doubled 

 by means of binding, but after that the possibility of regulation decreases, 

 and disappears altogether when the gastrula is completely formed. We must 

 here pass over the details of the minutely planned and carefully carried-out 

 experiments by which these statements are proved, as also the particulars 

 of the various attempts he made to get mechanical influences to operate — 

 pressure, binding up, and such means, whereby different observers have 

 sought to trace out the forces operating inside the egg and to discover the 

 details of which they are composed. 



Besides these purely mechanical experiments others have, of course, been 

 carried out, such as those in which eggs and embryos have been subjected 



