EXPERIMENTAL MORPHOGENESIS 63 



the cleavage- stages without any damage to the resulting 

 organism. There may be no micromeres at the sixteen-cell 

 stage, or they may appear as early as in the stage of eight 

 cells ; no matter, the larva is bound to be typical. So it 

 certainly is not necessary for all the cleavages to occur 

 just in their normal order. 



But of greater importance for our purposes was what 

 followed. I succeeded in pressing the eggs of Echinus 

 between two glass plates, rather tightly, but without killing 

 them ; the eggs became deformed to comparatively flat 

 plates of a large diameter. Now in these eggs all nuclear 

 division occurred at right angles to the direction of pressure, 

 that is to say, in the direction of the plates, as long as 

 the pressure lasted ; but the divisions began to occur at 

 ri^ht angles to their former direction, as soon as the 



o o 



pressure ceased. By letting the pressure be at work for 

 different times I therefore, of course, had it quite in my 

 power to obtain cleavage types just as I wanted to get 

 them. If, for instance, I kept the eggs under pressure 

 until the eight-cell stage was complete, I got a plate of eight 

 cells one beside the other, instead of two rings, of four 

 cells each, one above the other, as in the normal case; but 

 the next cell division occurred at right angles to the former 

 ones, and a sixteen-cell stage, of two plates of eight cells 

 each, one above the other, was the result. If the pressure 

 continued until the sixteen-cell stage was reached, sixteen 

 cells lay together in one plate, and two plates of sixteen 

 cells each, one above the other, were the result of the next 

 cleavage. 



We are not, however, studying these things for 

 cytological, but for morphogenetical purposes, and for these 



