CELL DIVISION IN EGGS OF CREPIDULA. 547 



there would be ground for establishing a fundamental distinction between these 

 two types of eggs; but the fact that any one of many slight changes in the environ- 

 ment may cause a non-parthenogenetic egg to become parthenogenetic indicates that 

 no such distinction can be of a very fundamental nature, and it also indicates that 

 there is no inherent improbability in the view that cleavage centrosomes may in some 

 cases arise from the egg centrosome as well as from the sperm centrosome. 



Kostanecki believes that my views are based upon a too narrow consideration 

 of the case of Crepidula. I, on the other hand, believe that the view that the 

 cleavage centrosomes always come from the sperm centrosome is based upon a 

 too narrow view of the conditions in certain fertilized eggs, with a corresponding 

 neglect of the conditions in normally and artificially parthenogenetic eggs. 1 

 I cannot better sum up my present views on this subject than by quoting the 

 conclusions reached in a former paper (1902, p. 30), to which I still adhere: 

 ''When one looks at the problem of fertilization from a general point of view, 

 when one considers the universality of sexual reproduction, when one reflects 

 upon the multitudes of exquisite adaptations which exist for securing the union 

 of egg and sperm, he will be loath to believe that the essential feature in fertili- 

 zation is the addition of a centrosome to the egg cell, or the supplying of a stimulus 

 to its development, which is not needed in all cases and which can as well be 

 supplied by changes in density, salinity, temperature, etc., as by the entrance of 

 a spermatozoon." 



7. Irregularities in the Movements of Chromosomes. 



In the separation of daughter chromosomes great irregularities appear; 

 "lagging" chromosomes are almost always present and in many cases the 

 chromosomes are scattered along the whole length of the spindle (figs. 211-222). 

 As a result of these irregularities of movement on the part of the chromosomes 

 they are not distributed equally to the two poles of the spindle and they do not 

 come together at the poles to form a single nuclear vesicle, but they become 

 associated in varying numbers to form vesicles, or karyomeres, of different sizes, 

 depending upon the number of chromosomes which enter into them, while if scat- 

 tered along the whole length of the spindle, the chromosomes may form a con- 

 tinuous chromatic connection between daughter nuclei, thus giving rise to con- 

 ditions which are falsely suggestive of amitosis. The karyomeres may be few 



1 Wilson (1897) demonstrated that in echinid eggs the entire middle piece of the spermatozoon was 

 not involved in the formation of the sperm centrosome. Meves (1911) has shown that no portion of the 

 middle piece is necessary for the formation of the sperm centrosome. F. R. Lillie (1912) in a recent and 

 most important paper has shown that in Nereis both middle piece and tail of the spermatozoon remain 

 outside the egg and that the sperm centrosome arises from the base of the sperm head. Furthermore 

 portions of the elongated sperm nucleus may be stripped off from the egg along with the jelly membrane, 

 by centrifugal force, and in such cases the sperm centrosome always arises from the basal end of that portion 

 of the nucleus which enters the egg, and is proportional in size to the quantity of nuclear material which 

 thus enters. He concludes "that the centrosome and aster owe their existence to an interaction between 

 nucleus and cytoplasm and not to any third element." The view that the cleavage centrosome can come 

 only from the centrosome of the spermatozoon is evidently losing ground. 



