22 



NORMAL HISTOLOGY. 



of external appendages, as the cilia of many unicellular organisms 

 or of the epithelial elements of the higher animals. 



Since every cell is derived from a pre-existing cell, it follows that 

 all the cells of the organism are the descendants of the parent ele- 

 ment the ovum. The ripe mammalian egg, while small in com- 

 parison with many other ova, is among the largest histological 

 elements, measuring about .2 millimetre in diameter, and, further, 

 possessing all parts of the typical cell. 



Before the ovum is capable of uniting with the male sexual element 

 to carry out the changes attendant upon fecundation, it passes through 

 a cycle of preparatory stages collectively known as maturation. 

 These changes consist in the repeated very unequal division of the 

 ovum, resulting in the expulsion of minute portions of its proto- 

 plasm, the polar bodies ; of these latter, usually two are extruded. 



Maturation and fecundation in ova of ascaris megalocephala : I,n, nucleus of ovum before matura- 

 tion ; s, entering spermatozoon ; //, nucleus () has passed to periphery of cell preparatory to di- 

 viding; s, spermatozoon now within the ovum; ///.nucleus dividing into first polar body (/); m, 

 male pronucleus resulting from spermatozoon ; IV, p, p 1 , first and second polar bodies, the last still 

 in process of formation ; m, male pronucleus ; V, p, p> ', polar bodies ; f and m, respectively female 

 and male pronuclei, in contact but not yet fused ; c, centrosomes, indicating poles of nuclear spindle ; 

 VI, pronuclei now fused ; striation proceeds from centrosomes preparatory to division of ovum. (After 

 O. Hertwig.) 



The nucleus which appears within the ovum after the formation of 

 the polar bodies is the female pronucleus. Upon the completion 

 of these phenomena, maturation has taken place and the ovum is 

 prepared for the reception of the male sexual element. Under 



