On the basis of experiments on ova which had been 

 pressed in water, Warnek concluded that the structural 

 changes in the nuclei depend upon the change of their 

 chemical composition. The chemical changes that appear, 

 in Warnek' s opinion, condition also the further transforma- 

 tions in the developing ova. After that when the fissure 

 begins in the dorsal side (Figure 40, 14), it passes around 

 half the periphery of the yolk; also in the ventral side, 

 a deepening appears (Figure 40, 15) and the yolk is twisted 

 by the meeting fissures. It acquires first the form of a 

 biscuit, and then two united or even completely isolated 

 globules (Figure 40, 17) . At the time of division of the 

 yolk the light spot (nucleus) divides into two parts, each 

 of which at first have caudiform processes, directed to the 

 point of contact of the yolk globules (Figure 40, 16) . The 

 processes quickly disappear, and the spot becomes spherical. 



These phenomena, according to Warnek, characterize 

 the first half of the second stage of division. At this 

 time it is not possible to isolate the nuclei by (pressing) 

 the yolk globules; from which it must be concluded that the 

 nuclei are still deprived of membranes, 



The second half of the second stage begins with the 

 dividing globules moving nearer. Between them a noticeable 

 cavity, formed from a transparent substance, emerges . Warnek 

 considered this transparent substance the product of separa- 

 tion of the yolk globules. At the time the dividing globules 

 are moving closer, the contours of the nuclei in them again 

 become clear, i.e. the membrane appears (it is clearly 

 noticeable in Li-max) by strongly refracting the color of the 

 nucleolus. In Lymnaeus stagnalis the nuclei, at first, are 

 situated near each other (Figure 39, 22), then separate, 

 sink in the depth of the yolk globules, and at the end 

 become invisible (Figure 39, 25) . Following the described 

 phenomena each of the two yolk globules become pear-shaped. 

 This outlines the passage to the third stage of division, 

 during which the yolk is divided into four parts. 



THE THIRD STAGE begins with changes in the nuclei, 

 acquiring a biscuit or figure-eight shape (Figure 39, 25a); 

 in the drawing it is seen that their division is not 

 accomplished simultaneously: when one divides, the other 



551 



