Ookinesis in (Vrebratiiliis Laeteus. 375 



largenieiit of tlie ceiitrosoine takes place. The direction of its growth 

 is not determined by the centrioles, but by the general organization 

 of the egg (cf. Mark, '81, p. 526, Boveri, '00, pp. 48, 107).''' 



Then the centrioles move apart from each other in the centrosome 

 (which differs from Coe's observation, '99, p. 459). The separation 

 seems in some way correlated with the growth of the centrosome. 

 As the centrioles move apart two dark staining fibres can be seen 

 between them (PL III, Fig. 48), bnt no central spindle (netrum) 

 is present (Schreiners, '06, p. 331, Fig. 179). One of the con- 

 necting fibres may remain a little longer than the other (PI. Ill, 

 Fig. 51). The centrioles then take their definitive position, the line 

 connecting two centrioles being perpendicular both to the vertical 

 egg axis and to the spindle. Their position is, as already mentioned, 

 governed by the shape of the centrosome, which is in turn depen- 

 dent on the general organization of the egg. The centrosome is 

 alveolar in structure (Vejdovsky, '88, p. 19, Wilson, '99) and has 

 the form of a sausage flattened horizontally. It now grows very 

 rapidly, partly at the expense of the chromosomal fibres, partly by 

 modification of the archiplasm. The chromosomal fibres shorten 

 without thickening, which suggests the hauling-in of a rope as Rhum- 

 bler states in the case of the polar rays ('96, p. 607), the fibres 

 being gradually metamorphosed into the centroplasm (Wilson, '95, 

 p. 2, '96, p. 77, '01, p. 387; Coe, '99, p. 459). The chromosomal 

 fibres completely disappear, and the chromosomal vesicles are found 

 actually in the centroplasm. The centrosome is now a flat sheet 

 and the centrioles have moved to a point near the outer periphery 

 of the centrosome. Here they acquire a ray system but are at first 

 devoid of the centroplasm. Then the old centrosome begins to dis- 

 integrate. Hand in hand with this dissolution the rays around the 

 centrioles become more and more distinct; often a spindle may be 

 formed as the result of secondary connection of the rays. As the 



"'In this coiiiiectioii it is interesting to mention that in the egg of Ar'hacia 

 r-entrifugalizert anrl afterward fertilized, the second cleavage takes place 

 always hoiMzontally. :is I was told l»y Professor Morgan. I think in the eggs 

 thns treated n vei-lic-il movement of the egg material (which tends to restore 

 the original structure) controls the definite shape of the first cleavage cen- 

 trosome and this in turn the position of the centrioles for the next cleavage. 



