THE FERTILIZATION OF THE OVUM. 



21 



spermatozoon enters the egg until a late stage in the cleavage 

 of the fertilized ovum. In the main, my observations on the 

 fertilization confirm those of Fol, concerning which a good 

 deal of doubt has been expressed by some authorities, though 

 in some respects they differ from these and resemble more 

 closely the results obtained by Guignard. 



Since the aster plays so important a part in the process of 

 fertilization, it will be well to begin with a description of this 

 structure. Every aster undergoes periodical changes in form 

 and size as well as constitution. When it has reached its 

 largest size, which is just before it divides, it is a spherical or 

 ellipsoidal body, almost as large as the nucleus, Fig. 2. It 

 has a very definite out- 

 line from which radiating 

 rows of granules or micro- 

 somes can be traced to all 

 parts of the cell. The 

 sphere itself consists of 

 an outer, darkly granular 

 zone and of a central clear 

 area. The > granules of 

 the outer zone can in no 

 way be distinguished from 

 the microsomes scattered 

 throughout the cell ; their 

 general appearance is very 

 similar and they stain in 

 the same way. Moreover, at certain stages in the history 

 of the aster it loses its definite outline, astral radiations 

 proceed from it in every direction, and the granules of 

 which it is composed become confluent with the microsomes 



"para-nuclei," etc., I have decided to employ the name "aster," first used by 

 Fol, I believe. This name has the advantage of brevity, simplicity and accuracy, 

 which cannot be said of the others. Some of these names are not only unwieldy, 

 but absolutely misleading. For example, the spheres attractive* have the function 

 of repulsion as well as attraction ; the spheres directrices do not, in all cases at least, 

 direct the nuclear division, and the same may be said of the archoplasm or "con- 

 trolling" plasm. The exact connotation of the words "periplast" and "para- 

 nucleus" is so doubtful that they cannot safely be used. 



FIG. 2. Section of one of the first four 

 micromeres of C. plana showing the structure 

 of the aster and its relations to other parts of 

 the cell. 



