CELL DIVISION IN EGGS OF CREPIDULA. 539 



suppressed and cytokinetic movements and division of the protoplasm is stopped 

 or greatly retarded. Figs. 159-170 represent eggs which were placed in 1 per 

 cent. NaCl in sea water for 4 hrs. and were then fixed at once. In all of the 

 eggs shown on plate LV nuclear and centrosomal division is going on, though 

 yolk cleavage has been suppressed. 



Other cases, in which cleavage has been suppressed in the yolk while still 

 going on in the protoplasm, nuclei, and centrosomes, are shown in figs. 136-208 

 and 209-223. In all of these cases the eggs were subjected to a weaker solution 

 for a long time or to a stronger solution for a short time, after which they wer< 

 returned to normal sea water. In all these eggs, which are in division stag< 

 multipolar spindles are present, which are probably the result of the interference 

 of originally separate spindles; while multiple nuclei and spheres are present in 

 resting stages. Cleavage is limited entirely to the protoplasmic portion of 

 the egg, which is thus transformed from a holoblastic to a meroblastic type 

 and it is interesting to observe that in many cases the micromeres formed show 

 more or less resemblance to normal micromeres. Thus in fig. 211 the spindl 

 are in position for the formation of the first set of micromeres; in fig. 213 a first 

 set of micromeres, normal except in number of cells and nuclei, is present, and in 

 fig. 212 a first and second set are present, which are also normal, with the excep- 

 tions just specified. Similar tendencies to normal micromere formation, after 

 the suppression of the first or second cleavages of the yolk, are shown in figs. 214- 

 218, et seq. In all of these cases the phenomena of nuclear division are very 

 abnormal and the fact that micromeres may be formed in more or less normal manner 

 in such eggs indicates that the form of cell division may be to a certain extent inde- 

 pendent of the type of nuclear division. 



Boveri (1897) found that when the first cleavage furrow in the egg of Echinus 

 is suppressed by pressure it never reappears; two cleavage nuclei are left in the 

 undivided egg and at the second cleavage two spindles are formed, usually vertical 

 to the axis of the first cleavage spindle and parallel to each other; the second 

 cleavage furrow then appears as if it were the first and two cells are formed each 

 with two nuclei, and this process is continued in later cleavages. Wilson (1901 J ), 

 on the other hand, found that in such eggs of Toxopneusks the first cleavage 

 furrow was restored after the third cleavage and sometimes even earlier. All 

 my observations on Crepidula indicate that when once a cleavage furrow has been 

 suppressed and other furrows are subsequently formed, the suppressed furrow i* 

 never restored. 



2. Suppression of Division in both Yolk and Protoplasm without fib Suppression 



in Nucleus and Centrosome. 



(Figs. 174, 175, 183-190.) 



In salt solutions of medium strength (1 per cent. KC1, 2 per cent. NaCl 



3 per cent, to 4 per cent. MgCl 2 ) cleavage of both yolk and protoplasm 



* 





