546 



CELL DIVISION IN EGGS OF CREPIDULA. 



Crepidula. 



Egg with abundant yolk. 



Large centrosomes and asters in 

 both maturation spindles. 



Egg centrosome remains evident for 

 some time and then disappears in egg 



sphere. 



Egg aster gives rise to large sphere 



which persists till germ nuclei conju- 

 gate. 



Sperm nucleus moves very slowly 



Ciona. 



Egg with little yolk. 



No centrosomes or asters in either 

 maturation spindle. 



No egg centrosome at any stage. 



No egg aster or sphere at any stage. 



Sperm nucleus and egg nucleus 



toward egg nucleus; first cleavage is move rapidly toward posterior pole; 



first cleavage is about 40 min. after 

 fertilization. 



Sperm nucleus, centrosome and 



about 6 hrs. after fertilization 



Sperm nucleus, centrosome and 



aster lie for 3 or 4 hrs. in compact yolk aster lie in cytoplasm from the first 



and remain small 



Sperm centrosome does not divide 



and grow rapidly. 



Sperm centrosome divides giving 



but disappears in sperm sphere which rise to sperm amphiaster. 



persists till germ nuclei conjugate 

 Egg and sperm spheres fuse and dis 



Sperm amphiaster becomes the 



integrate when germ nuclei 



gate, central spindle of the first cleavag 



and one cleavage centrosome appears 

 close to each nucleus, and the cleavage 

 spindle forms between them. 



When to these differences in the normal phenomena of fertilization is added the 

 fact that hypertonic sea water may cause a perfect spindle to form in connection 

 with each of the germ nuclei in Crepidula but that no means has yet been found, 

 though many have been tried, to cause a spindle to form in connection with the 

 egg nucleus of ascidians, the extent of the differences between the eggs of these 



two types will be appreciated. . . 



The view that this difference ceases when we come to the actual origin oi 

 the cleavage centrosomes is based not upon actual evidence but upon the con- 

 viction that a single, uniform origin of these centrosomes must be expected 

 v „„ v ^ upV v of all other differences there must be uniformity in this regard. 

 And yet nothing is more certain than that the cleavage centrosomes do not have 

 uniform origin in all cases, that while, in many instances they nrtn " 



that 



spite 





nection with the sperm nucleus, in all cases 



of normal or artificial partheno- 



genesis they cannot have such an origin 



There is not therefore a single, urn 



form origin of these centrosomes in all animals, and consequently there is no anie 



dent probability that the differences observable between Crepidula and Ctojw, 



example, with respect to other phenomena of maturation and fertilization 



not also extend to the origin of the cleavage centrosomes 



If it were certain 



that the cleavage centrosomes invariably come from the sperm centros .°^ 

 fertilized eggs, and invariably from some other source in parthenogenetic & 



! 



