60 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



experiment on another subject and by a somewhat different 

 method. It was known from the cytological researches 

 of the brothers Hertwig and Boveri that the eggs of the 

 common sea-urchin (Echinus microtulerculatus) are able to 

 stand well all sorts of rough treatment, and that, in 

 particular, when broken into pieces by shaking, their frag- 

 ments will survive and continue to segment. I took 

 advantage of these facts for my purposes. I shook the 

 germs rather violently during their two-cell stage, and in 

 several instances I succeeded in killing one of the blastb- 

 meres, while the other one was not damaged, or in separat- 

 ing the two blastomeres from one another. 1 



Let us now follow the development of the isolated 

 surviving cell. It went through cleavage just as it would 

 have done in contact with its sister-cell, and there occurred 

 cleavage stages which were just half of the normal ones. 

 The stage, for instance, which corresponded to the normal 

 sixteen-cell stage, and which, of course, in my subjects was 

 built up of eight elements only, showed two micromeres, two 

 rnacrorneres and four cells of medium size, exactly as if a 

 normal sixteen-cell stage had been cut in two ; and the form 

 of the whole was that of a hemisphere. So far there was 

 no divergence from Koux's results. 



The development of our Echinus proceeds rather rapidly, 

 the cleavage being accomplished in about fifteen hours. I 

 now noticed on the evening of the first day of the experiment, 

 when the half-germ was composed of about two hundred ele- 

 ments, that the margin of the hemispherical germ bent together 

 a little, as if it were about to form a whole sphere of smaller 

 size, and, indeed, the next morning a whole diminutive 



1 Zcitschr. wiss. Zool. 53, 1891. 



