CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 369 



10. The spongioplasni of the egg of Crepidula is the interal- 

 veolar or continuous substance within which are found enchy- 

 lemma, microsomes, mitochondria, as well as yolk, oil and 

 other inclusions. The form taken by this otherwise continuous 

 substance depends largelj^ upon its relation to these other in- 

 cluded substances and it does not consist of preformed fibers or 

 other structures which are definite in number and position. It 

 is most abundant at the animal pole of normal eggs from which 

 it radiates as a spongework between the other inclusions growing 

 more and more coarse as it approaches the vegetal pole (pp. 

 369-376). 



11. Protoplasmic flowing and intracellular movements are 

 probably caused by the contractility of the spongioplasni. It 

 contracts to points of stimulation, such as the entrance point 

 of the spermatozoon, the centrosomes of mitotic figures, etc. 

 Very small inclusions, such as pigment, may be carried along 

 with the spongioplasni in its contraction; larger inclusions such 

 as yolk spheres are forced out of the regions where spongioplasni 

 concentrates (pp. 376-378). 



12. The orientations of development such as polarity, sym- 

 metry, localization of inclusions, pattern of cleavage, etc. are 

 largely determined by the structure and activities of the 

 spongioplasni, which probably differ in different parts of the 

 egg and at different stages of development (pp. 378-381). 



13. The division plane between daughter cells always passes 

 through the equator of the mitotic spindle and at right angles to 

 its axis. If both poles of the spindle are attached to the 

 periphery of the cell it cannot be moved except by very violent 

 centrifuging ; if one pole is attached the other may be deflected 

 to one side or the other; if neither pole is firmly attached the en- 

 tire spindle may be moved. In normal maturation divisions 

 one pole of the spindle is attached to the periphery at the ani- 

 mal pole of the egg and during division this aster is absorbed 

 into the peripheral layer, the spindle grows very short and when 

 the division wall forms through the equator of the spindle it 

 cuts off a minute polar body from a relatively enormous egg 

 (p. 336). 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 22, NO. 2 



