222 EDWIN G. CONKL1N. 



ent animals or even in the same animal under different condi- 

 tions. 



The experiments of R. Hertvvig, Morgan, Loeb and Wilson 

 have shown that the unfertilized egg is capable of giving rise to 

 cleavage centrosomes and spindles, while the observations of 

 Delage, Boveri and others on merogeny have proven that a nor- 

 mal mitotic figure may appear in connection with the sperm 

 nucleus in enucleated egg fragments. Under these circumstances 

 it ought to be possible, by slightly altering normal conditions, to 

 bring about the formation of a spindle in connection with each 

 germ nucleus. 



In the summer of 1901, while at the Marine Biological Labora- 

 tory at Woods Holl, I undertook some experiments on the eggs 

 of Crepidula in order among other things to test this possibility. 

 Since this animal is one which does not conform to the prevalent 

 view as to the origin of the cleavage centrosomes it seemed all 

 the more favorable for such work. As the eggs of these gas- 

 teropods are fertilized while still in the oviduct I have found it 

 impracticable to experiment with unfertilized eggs but have 

 worked entirely with eggs into which a spermatozoon had already 

 entered. 



The normal course of the fecundation in this animal may be 

 briefly recalled : After both polar bodies have been extruded the 

 egg nucleus lies at the animal pole in an area of cytoplasm while 

 the sperm nucleus lies in the yolk near the periphery of the egg 

 and usually near the vegetative pole. Then the egg centrosome 

 which is left in the egg at the close of the second maturation 

 division, rapidly disappears and in its place is left the large egg 

 aster or sphere. At no stage is there a clearly marked sperm 

 centrosome, but a radiating cytoplasmic figure, the sperm aster, 

 develops in connection with the sperm nucleus and these two 

 migrate through the yolk until they come into contact with the 

 egg nucleus and sphere, immediately under the polar bodies. 

 Here the egg and sperm spheres fuse and at their periphery two 

 separate and independent centrosomes appear which ultimately 

 come to lie at opposite poles of the first cleavage spindle. It is 

 probable that one of these centrosomes comes from the egg 

 sphere and the other from the sperm sphere though there is no 



