ROBERT CHAMBERS 61 



undisturbed. As soon as the amphiaster disappeared there was no 

 longer a tendency for the furrow to form (Fig. 8 a). The two nuclei 

 now lay in a fluid cytoplasm. Within half an hour after the suppres- 

 sion of the first segmentation furrow, an amphiaster developed about 

 each nucleus preparatory to the next division. The two amphiasters 

 lay side by side but remained distinct from one another, no connecting 

 radiations being formed (Fig. 8 6). The formation of the two amphi- 

 asters resulted in the transformation of the egg substance into four 

 semirigid bodies, the four asters. Cleavage furrows now extended 

 into the fluid regions between the asters and divided the egg almost 

 simultaneously into four blastomeres. The furrow corresponding to 

 the second cleavage started to form and cut through the egg about a 

 minute ahead of that of the first (Fig. 8 c and d) . 



This experiment may throw light on the nature of the segmentation 

 in ova in which several nuclear divisions follow one another with no 

 outward manifestation of the segmentation of the egg. After a 

 certain period the ovum breaks up simultaneously into as many 

 blastomeres as there are nuclei. This is the normal method in certain 

 Actinozoa and can be artificially produced in many eggs by exposing 

 them to various reagents, notably hypertonic solutions.^ -'^ 



The solidification associated with the aster formation divides the 

 egg cytoplasm into a number of bodies each surrounding a nucleus. 

 Between successive divisions the cytoplasm reverts to a more fluid 

 state but its viscid nature may suffice in preventing the merging of 

 neighboring areas. After a varying number of nuclear divisions with 

 accompanying solidification periods furrows suddenly appear between 

 these bodies and the ovum tends to break up at once into separate 

 blastomeres. A differentiation of this type may possibly have taken 



®Loeb, J., Investigations in physiological morphology. III. Experiments on 

 cleavage, /. Morph., 1892-93, vii, 253. Norman, W. W., Segmentation of the 

 nucleus without segmentation of the protoplasm, Arch. Entwcklngsmechn. Organ., 

 1896, iii, 106. Wilson, E. B., Experimental studies in cytology. I, ibid., 1901, 

 xii, 529. Lillie, R. S., Fusion of blastomeres and nuclear division without cell 

 division in solutions of non-electrolytes, Biol. Bull., 1902-03, iv, 164. 



'Wilson, E. B., Experimental studies in cytology, II and III, Arch. Ent- 

 wcklngsmechn. Organ., 1902, xiii, 353. 



