The presence of vesicles where the formation of fissures 

 of division begins did not prove, in Warnek's opinion, that 

 the topographical position of the fissures was determined 

 by the vesicles. Preferably, as he thought, the matter was 

 the contrary: the vesicles are separated where the center 

 of division is present. In later stages of division many 

 fissures appear without preliminary separation of the vesicles 



After the separation of both vesicles the transparent 

 crescent-shaped region also disappears. In the ova of the 

 slug there appear two nuclei distinctly separated from each 

 other. At this time the nuclei each acquire distinct contour 

 and a large nucleolus. 



THE SECOND STAGE begins with the loss of the membranes 

 of the nuclei and their merging into one common mass. This 

 mass acquires an extended form, situated on the longitudinal 

 diameter of the yolk, i.e. at right angle to the position 

 which is characteristic for the first stage (Figure 40, 11) . 

 Then the nucleus becomes biscuit-shaped (Figure 40, 12) and 

 at the same time the division of the yolk begins. In the 

 last description Warnek used topographical terms, and their 

 significance is explained in the literal remark. The transverse 

 diameter he called the diameter; passing through the vesicles 

 from the yolk at right angle to it the longitudinal diameter 

 is situated. The terminal points of these diameters he called 

 poles: the dorsal pole in the place where the external vesicles 

 were present; the ventral pole situated against the dorsal one; 

 the poles of the longitudinal diameter designated as right and 

 left. 



The division of the yolk is preceded by a thickening of 

 the dorsal pole, in its field; then a fissure in the form of 

 a cut appears. Due to its deepening the ovum becomes in form 

 more like the kidney (Figure 40, 13) . The direction of the 

 fissure does not coincide with the transverse diameter; it 

 is inclined to it at a 45-degree angle. The light spot 

 decreases and becomes less noticeable even in the ova of the 

 slug, and in (Lymnaeus stagnali-s) it is not seen in the 

 majority of cases from the very beginning of the division. 



547 



