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- 



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 

 move rapidly toward posterior pole; 

 first cleavage is about 40 min. after 

 fertilization. 



Sperm nucleus, centrosome and 

 aster lie in cytoplasm from the first 

 and grow rapidly. 



Sperm centrosome divides giving 

 rise to sperm amphiaster. 



Sperm amphiaster becomes the 

 central spindle of the first cleavage. 



Sperm nucleus moves very slowly 

 toward egg nucleus; first cleavage is 

 about 6 hrs. after fertilization. 



Sperm nucleus, centrosome and 

 aster lie for 3 or 4 hrs. in compact yolk 

 and remain small. 



Sperm centrosome does not divide 

 but disappears in sperm sphere which 

 persists till germ nuclei conjugate. 



Egg and sperm spheres fuse and dis- 

 integrate when germ nuclei conjugate, 

 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 of 

 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, — 

 that in spite 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 

 a uniform origin in all cases, that while, in many instances they arise in con- 

 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, uni- 

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

 dent probability that the differences observable between Crepidula and Ciona, for 

 example, with respect to other phenomena of maturation and fertilization do 

 not also extend to the origin of the cleavage centrosomes. If it were certain 

 that the cleavage centrosomes invariably come from the sperm centrosome in 

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



