52 PROTOPLASMIC CONSISTENCY AND CELL DIVISION 



after maturation it drops. Upon fertilization it begins to rise again, 

 to reach its maximum with the full development of the sperm aster. 

 The viscosity drops again and continues low until the approach of 

 cleavage. It thereupon rises again to drop only after completion of 

 the first cleavage. Subsequent to the first cleavage the rhythmic 

 appearance and disappearance of the asters within the blastomeres 

 most probably indicate periodic successions of a process analogous 

 to a jellying and hquefying of the cytoplasm. 



The segmentation process may thus be explained as consisting 

 essentially in a growth within the egg of two bodies of material 

 through a gradual transformation of the cytoplasm. This transfor- 

 mation is associated with a change in the physical state of the proto- 

 plasm, two semisoHd masses growing at the expense of the more 

 fluid portions of the cytoplasm. 



//. Cutting Experiments on the Segmenting Egg. 



If it is true that the segmenting egg consists of two rather firm 

 masses which are most fluid at their periphery, and if the physical 

 state of the protoplasm is not affected in the process, one should be 

 able to cut a segmenting egg into pieces without disturbing the 

 cleavage plane. Cleavage should, therefore, proceed in such a manner 

 as to complete the separation of what remains of the two bodies 

 within each piece. This is what actually happens. Some experi- 

 ments of Yatsu,^ the results of which he made no attempt to explain, 

 are in full accordance with mine and bear directly on this problem. 

 Yatsu cut the eggs of Cerebratulus which were just beginning to 

 segment (anaphase stage) into nucleated and non-nucleated frag- 

 ments. He found that the cleavage furrow proceeded in its original 

 plane irrespective of whether the fragments were nucleated or not. 

 In Fig. 1, I have diagrammatically presented some of his results. 

 Fig. 1 a represents a segmenting Cerebratulus egg being cut in a plane 

 parallel to its long axis and to one side of the daughter nuclei. The 

 original furrow persisted in the non-nucleated fragment (b) and 



* Yatsu, N., Some experiments on cell-division in the egg of Cerebratulus 

 lacteus, Annot. zool. japon., 1908, vi, 267. 



