FERTILIZATION OF THE EGG. 79 



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

 zation, but is merely a locomotor apparatus for the head (nucleus) 

 and middle-piece. 



Within the ovum the head of the spermatozoon persists as 

 the sperm-nucleus (or male pro-nucleus), while the protoplasm in 

 its neighborhood assumes a peculiar and characteristic radiate 

 arrangement like a star, probably through the influence of the 

 middle-piece. 



After the entrance of the spermatozoon the egg segments off 



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 spermatozoon; 

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

 trosome near it (modified from Hertwig). C, 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 nucleus. 

 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 

 polar cells or polar bodies. These take no part in the formation 

 of the embryo, and their formation probably serves, in some way 

 -not yet wholly clear, to prepare the egg for the last act of 

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

 nucleus (now often called \h& female pro-nucleus) and the sperm- 

 nucleus approach one another and finally become intimately 



