CELL DIVISION IN EGGS OF CREPIDULA. 555 



suppression of cleavage in the yolk, with subsequent formation of polyasters and 

 meroblastic cleavage; scattering of chromosomes, with consequent formation of 

 karyomeres, chromatic connections between daughter nuclei, and amitosis-like 

 figures. (Figs. 132-158; pp. 532-536.) 



10. In sea water rendered hypertonic by the addition of KC1, NaCl, or 

 MgCl 2 , yolk cleavage is suppressed in weaker solutions, protoplasmic cleavage 

 in stronger solutions, nuclear and centrosomal divisions in still stronger solutions; 

 archiplasm, chromatin and mitotic figures shrink, and cytasters are formed from 

 isolated portions of archiplasm of the astral radiations; multiple nuclei and 

 multipolar spindles are due to the suppression of cell division while nuclear and 

 centrosomal divisions proceed; irregular movements of chromosomes lead to the 

 formation of isolated chromosomal vesicles or karyomeres; achromatin may 

 separate from chromatin and form achromatic vesicles with achromatic mem- 

 branes. (Figs. 159-223; pp. 536-550.) 



B. Cleavage and Differentiation. 



11. Early stages in development are more easily influenced by environmental 

 changes than are later ones; and stages during ldnesis are more susceptible to 

 modification than stages during interkinesis. Almost all persistent alterations of 

 structure occur during cell division, few of those which occur during the resting 

 period are permanent. That this is due primarily to interference with the divi- 

 sional apparatus rather than to increased permeability of the plasma membrane 

 during division is indicated by the facts (1) that this is true of eggs subjected to 

 pressure and to the electric current as well as to the various solutions used; 



(2) that weak solutions produce more changes of a permanent character when 

 acting during kinesis, than do much stronger solutions acting during interkinesis; 



(3) that practically all persistent modifications of structure are those which affect 

 the division of nucleus, centrosome or cell body, whereas great modifications of 

 structure during the resting period quickly disappear when the eggs are returned 

 to normal conditions. The cause of this is evidently to be found in the fact that 

 when chromosomes, centrosomes or differentiated portions of the cell body are 

 once distributed abnormally to the two daughter cells there is no possibility of 

 bringing about a normal redistribution of these structures, owing to the inter- 

 vention of the division wall. (Pp. 537, 538.) 



12. The study of the development of eggs under pressure, and of isolated 

 blastomeres, shows that there is a polar-bilateral organization of the egg, that 

 the early cleavages are typically differential, and that the prospective significance 

 and the prospective potency of the early blastomeres are identical. On the other 

 hand the nuclear divisions are non-differential and the early nuclei totipotent. 

 (Pp. 515-518.) 



13. The causes of differential cleavage are to be found in the relations of the 

 spindle axes and the cleavage planes to the organization (differentiation and 

 localization) of the cell substance. The position of the mitotic figure in a cell is 



