242 



INTRODUCTION TO CYTOLOGY 



a process which Heilbrunn has associated with a lowering of surface 

 tension. ^1 These changes do not depend upon the actual entrance of the 

 spermatozoon into the egg ; in Nereis they occur before the slow penetra- 

 tion can be completed, or even if the spermatozoon is shaken loose shortly 

 after penetration has begun. In some animals (frog) the amount of 

 fluid in the "perivitelline space" under the membrane is great enough to 

 permit the rotation of the egg within it. 



In describing the remarkable transformation undergone by the sper- 

 matozoon within the egg it will be convenient to deal with its various 

 parts separately. 



The Nucleus. — Immediately after gaining entrance to the egg (Fig. 

 147), the sperm head begins to enlarge and assume the usual form and 



Fig. 147. — Diagram of syngamy and cleavage in an animal. It is assumed in this case 

 that meiosis has been completed before the entrance of the spermatozoon. 



structure of a nucleus. Meanwhile it advances toward the egg nucleus. 



As Lillie points out, both nuclei pass toward a position of equilibrium in a 



cell preparing to divide and consequently meet; the assumption of an 



attractive force between them is unnecessary. By the time they meet, 



the sperm nucleus has usually, but not always, become equal in size and 



appearance to the egg nucleus. The union of these two "pronuclei" 



usually occurs at once. In a great many cases there may be no actual 



fusion of the nuclei as such. As they come close to one another each 



passes through the prophase stages and gives rise independently to its 



group of chromosomes, the two groups arranging themselves in a common 



spindle which organizes as the nuclear membranes disappear. The 



nuclei may behave variously in this respect within the same species. In 



Strongylocentrotus lividus, Danchakoff (1916) observed three general 



" Heilbrunn (1913, 1915, 1924c, 19256). See also Peterfl (1927), Hobson (1927), 

 and Chambers (1930a) on the fertilization membrane; also the papers of Just (1920 

 et seq.). 



