THE BIOLOGY OF THE CELL SURFACE 



subsequently at each instant in the division-process the 

 same change in contour and exhibiting a clock-like precision 

 in the cleavage-rhythm. 



Another advantage inheres in the use of sea-urchins' eggs 

 for the investigation of cell-division. Because of the fact 

 that these eggs are fertilized after complete maturation, the 

 process of division is most easily observed, being that of a 

 single division-cycle and not imposed upon the events of 

 one or both maturation divisions as in eggs of the other three 

 fertilization-types. Moreover, the cleavage is of the most 

 simple type inasmuch as the first three cleavages form 

 almost equal and close to spherical cells. Since the first 

 cleavage cycle encompasses the union of the egg- and sperm- 

 nuclei while all succeeding cleavages are wholly mono-nu- 

 clear — a fact which we must ever bear in mind — it would 

 appear necessary to appreciate the sequence of events in 

 the second or the third cleavage in order properly to evalu- 

 ate those in the first in which part of the events belongs to 

 the fertilization-process. One must be fully cognizant of 

 what events belong to fertilization and what to cell-division. 

 To obtain a clear picture of the end of the one and the 

 beginning of the other process is not easy since so much of 

 the work on cell-division considers the first division only. 



The account now given is based upon the Qgg of Arbacia 

 because of its widespread use by investigators, and not 

 upon the more favorable but more sensitive egg of Echi- 

 7iarachnius, equally familiar to me but not so extensively 

 employed by others. My experience with these eggs as 

 well as with those of four different species of sea-urchins at 

 Naples convinces me that, despite certain structural dif- 

 ferences and some variations in their response to experi- 

 mental treatment — due to habitat, as, for example, 

 temperature and salinity of the sea-water — what I here set 

 forth applies generally to echinid eggs. In order to make 

 comparisons with the work of others who followed the 



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