274 



IN TRODVC TION TO C Y TOLOG 1 ' 



which grows out from the centrosome in the middle piece or in some 

 cases apparently from the base of the nucleus, and a cytoplasmic sheath 

 which usually extends not quite to its end. The sheath sometimes has 

 the form of an undulating membrane. The spermatozoa of crustaceans 

 and nematodes are non-flagellate, and in other groups various departures 

 from the " typical" form and structure are known. A few of the many 

 known types are shown in Fig. 103. 



G 



Fig. 103. — Various types of spermatozoa. 

 A, Triton (salamander). (After Ballowitz.) B, Nereis (annelid). (After Lillie, 1912.) 

 C, guinea pig. (After Meves.) D, Phyllopneuste (bird). (After Ballowitz.) E, sturgeon 

 (After Ballowitz.) F, Vesperugo (bat). (After Ballowitz.) G, Castrada hofmanni (turbel- 

 larian). (After Luther.) H, Pinnotheres veterum (crustacean). (After Koltzoff.) I,Homa- 

 rus (lobster). (After Herrick). J, Ascaris (nematode); a, apical body; n, nucleus; r, "re- 

 fractive body." (After Scheben.) 



The ovum undergoes nearly or quite all of its elaborate differentiation 

 before the maturation divisions occur. Certain cells in the ovary gradu- 

 ally become greatly enlarged (Fig. 105), and during this "growth period" 

 the cytoplasm may not only differentiate into visibly distinct regions 

 but may also become stored with energy-containing materials ("food"), 

 which in the case of some animals, such as birds, is present in relatively 

 enormous amounts. The "ovarian egg" or primary oocyte, as the egg 

 cell is called before the maturation mitoses take place, may have a definite 

 limiting membrane at its surface, but in many forms this cannot be 

 demonstrated. The nucleus of the primary oocyte is known as the 



