FERTILIZATION OF THE EGO. 



79 



It is probable that the tail plays no part in the actual fertili- 

 zation, but is merely a locomotor apparatus fur tlie head (nucleus) 

 and middle-piece. 



Within the ovum the head of the spermatozoun })ersi8ts as 

 the s^erin-nucleus (or male pro-nucleus)^ while the protoplaMn in 

 its neighborhood assumes a peculiar and characteristic ra<liate 

 arrangement like a star, probably through the influence of the 

 middle-piece. 



After the entrance of the spermatozoon the ^g'g segments ulf 



Fig. 34.— Fertilization of the ovum. A., entrance of the spermatozoon (in the sea- 

 urchin, after Fol). B, the sea-urchin egg after entrance of the sperniatozoim ; 

 within and to the left is the egg-nucleus; above is the sperm-nucleus, with a cen- 

 trosomenear it (modified from Hertwig). r, diagram of the ovum after extrusion 

 of the polar cells (p.c), and union of the two pro-nuclei to form the segmenta- 

 tion-nucleus. The smaller and darker portion of the latter is derived from the 

 sperm-nucleus. Two asters or archoplasm-spheres are shown near the nuiU-us. 

 These arise by the division of a single aster derived from the middle-piece of the 

 spermatozoon. D, two-celled stage of the earthworm, after the first fission of 

 the ovum. (After Vejdovsky.) 



at one side two small cells, one after the other, known as the 

 ^olar cells oy jyolar hodles. These take no part in tlie formation 

 of the embryo, and their formation prol)ab]y serves, in some way 

 not yet wholly clear, to prej)are tlie egg for the lju>t act of 

 fertilization. After the formation of the polar cells the egg- 

 nucleus (now often called t\\.Q female pi'o-nucle us) and tlie sperm- 

 nucleus approach one another and iinally become intimately 



