EXPERIMENTS ON CLEAVAGE -~>~> 



into normal sea-water, I found that the manner of s 



tion changes in a remarkable way, according to the time the 



eggs had been in the concentrated sea-water. 



3. I fertilized eggs of sea-urchins at 9:30 in the morn- 

 ing, and at 9:43 a part of these eggs were put into sea-water 

 to which 2 g. of NaCl to 100 c.c. had been added. The rest 

 of the eggs remained in normal sea- water. I will call the 

 sea-water to which 2 g. of NaCl to 100 c.c. had been added 

 the concentrated solution, and the eggs which had been 

 exposed to it the plasmolyzed eggs. At 10:20, before any 

 segmentation even in the normal sea-water had taken place, 

 I took a lot of eggs out of the concentrated solution and 

 brought them back into normal sea- water. At 10:33 these 

 eggs began to segment. The segmentation was a normal 

 one, as only segmentation into two cells took place. At the 

 same time segmentation had taken place in nearly all of the 

 normal eggs. The only difference between the normal eggs 

 and the plasmolyzed eggs was that the former at 10:33 were 

 nearly all segmented, while of the latter only a small part had 

 undergone segmentation. Ten minutes later, however, every 

 second one of the plasmolyzed eggs was segmented, mostly 

 into two, exceptionally into four, segments. But now the 

 situation began to change. By this time the normal eggs 

 began to reach the four-cell stage, and now many of the 

 plasmolyzed eggs which had not yet segmented into two cells 

 began to segment into three or four cells at once, without 

 going through the two-cell stage at all. The cleavage took 

 place in this way, that at the same time, or shortly after each 

 other, spherical projections appeared on the surface of tin- 

 egg, which at first were coherent, but which soon, at the same 

 time or in quick succession, were separated. This kind of 

 segmentation seems to be identical with that which O. and 

 R. Hertwig observed under other circumstances, and have 

 described as Knospenfurchung? The further segmentation 



I O. AND R. HERTWIG, .7o(i//x<-//<- Zr/tsrlirift, Vol XX (1S87). 



