72 A. RICHARDS. 



latter animal (Conklin, '02) apply to a large extent to the develop- 

 ment of Planorbis. 



The progress of the division as seen in the living egg is briefly 

 as follows. The nucleus can be seen in the resting state as a 

 clear spherical area distinguishable from the rest of the cytoplasm 

 by its lighter color. As the spindle progresses the egg elongates 

 slightly in the axis of the spindle in preparation for division. 

 In the succeeding cleavage this elongation is more clearly 

 marked. 



After the furrow has separated the blastomeres, they round 

 up until they are almost spherical in form and the contact 

 surface forms only a narrow protoplasmic bridge between them. 

 This rounding off of the blastomeres is repeated after each cleav- 

 age, at least after the earlier cleavages, and suggests, as Holmes 

 points out, the result of some tension which is exercised at the 

 poles of the eggs. The tension soon appears to decrease, however, 

 for the blastomeres flatten against each other gradually as if they 

 were being drawn together, with the result that each assumes a 

 more or less spherical form and the contact surface becomes a 

 mere line between the eggs. This flattening is accomplished by a 

 rotation of the blastomeres toward the animal pole. The nuclei 

 and the asters thus come to lie near the animal pole and the 

 spindle is bent sharply (cf. Conklin, '02). Presently, after the 

 blastomeres have flattened against each other in the two-cell 

 stage, a "lenticular cleavage cavity" (Holmes) appears between 

 them, and its maximum is a criterion as to the progress of the 

 next stage. 



Since these stages occur with little variation and can be seen 

 rather clearly in the living condition, it is possible to expose the 

 egg to the X-rays at almost any time desired. 



During the resting stage the nuclei when viewed from the 

 animal pole lie very near the furrow. Thus the two nuclei of 

 the separate blastomeres are brought as close to each other as 

 possible. At a little later stage the nucleus in each blastomere 

 passes down toward the center. 



The cytoplasmic constriction cuts in deeper at the animal 

 pole on the first cleavage as Holmes has pointed out; this is the 

 general rule with eggs of this type. It holds true for the sue- 



