544 



CELL DIVISION IN EGGS OF CREPIDULA. 



ters, and finally polyasters in cases where the division of the cell body is inhibited 

 while that of the centrosome proceeds. / have been unable to find any evidence 

 whatever that centrosomes arise de novo in the eggs of Crepidula, or that cytasters 

 become the centers of mitotic nuclear division. 



It is a striking fact, and one not easily harmonized with the hypothesis 

 that centrosomes may arise de novo, or from the fragmentation of a single centro- 

 some into many, that the number of nuclear centrosomes in eggs in which cleavag 



was 



ppressed by hypertonic 



water is approximately proportional to the 



length of time during which the eggs have remained in normal sea water following 

 exposure to the hypertonic solutions. The maximum number of cytasters is 

 found while eggs are still in the hypertonic solution (figs. 183, 189, 190) whereas 

 in such eggs the nuclear asters are present in normal numbers. Only after such 



eggs have been returned to normal 



water for several hours during which 



time centrosomal division has been progressing while cell division remains sup 

 pressed do nuclear asters become numerous (figs. 203, 207, 219, et seq.). 



pparent 



pti 



to this rule are found in those 



The 



which the 



hypertonic solution (or other injurious condition) was not strong enough to stop 

 centrosomal division, although strong enough to suppress cell division, and these 

 exceptions are only apparent and really support the proposition that nuclear 

 asters and centrosomes arise by the division of preexisting asters and centrosomes. 

 Further evidence that the polyasters which occur in the eggs of Crepidula are 

 not derived from cytasters may be found in the fact that polyasters may be found 



at any stage 



cytasters are rarely found later than the 2-cell stage 



of the cleavage, whereas according to Konopacki and myself 



6. Origin of the Cleavage Centrosomes. 



(Plate LV.) 



All of the eggs represented in plate LV were placed in 1 per cent. NaCl 

 in sea water for 4 hrs. and were then fixed at once. In all of these figures both 

 polar bodies had been formed and the sperm had entered the egg before the 

 experiment began. The effect of the hypertonic sea water has been to delay 

 the movement of the sperm nucleus toward the egg nucleus and to stimulate tne 

 formation of an amphiaster in connection with the egg nucleus. The view was 

 first expressed by Boveri (1892), and has been widely accepted by other investi- 

 gators, that the egg centrosome is a decadent structure, which ultimately unae - 

 goes degeneration, while the two cleavage centrosomes arise in connection wi 

 the spermatozoon. In cases of normal or artificial parthenogenesis it is suppos » 

 that the cleavage centrosomes are derived from the egg centrosome 

 such cases becomes active, or thej 

 processes of maturation 



which 



Mv work on the normal 

 and fertilization in Crepidula (Conklin, 1902) lea me 



new formations 



processes 01 maturation anu lertmzauuu m ^#oy «*«*«* v~ ' tinnvrith 



to the conclusion that one of the cleavage centrosomes is formed in connecl ' the 

 each of the germ nuclei, though it was impossible to say that they came 



