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INTRODUCTION TO CYTOLOGY 



sists of a central hydrophilic substance representing the nucleus, a periph- 

 eral lipochromatic layer consisting of cytoplasm with lipides and lipopro- 

 teins, and a delicate outer membrane; a fatty layer surrounds the whole 

 cell. In Sepia, W. J. Schmidt (1928) concludes that the head, which is 

 anisotropic in polarized light, is composed largely of submicroscopic rod- 

 shaped micellae arranged parallel to the longitudinal axis. 



In many animals, particularly mammals, a ''middle piece" is more 

 distinctly differentiated than it is in insects, the spermatozoon therefore 

 appearing to have three main parts. In general it is more convenient to 



J 



Fig. 135. — Spermatozoa of various animals. A, Triton (salamander). B, Nereis 

 (annelid). C, guinea pig. D, Phyllopneuste (bird). E, sturgeon. F, Vesperugo (bat). 

 G, Castrada (turbellarian). H, Pinnotheres (crustacean). I, Homarus (lobster). /, 

 Ascaris (nematode): a, apical body; n, nucleus; r, "refringent body." (From the works of 

 Ballowitz, F. R. Lillie, Meves, Luther, Koltzoff, Herrick, and Scheben.) 



think of the middle piece as a portion of the tail. The tail of the typical 

 animal spermatozoon as a rule may be said to have three segments, 

 though in certain animals all of them may not be differentiated. These 

 are the "middle piece," in which the axial filament is enclosed in a sheath 

 of undifferentiated cytoplasm carrying the mitochondrial elements and 

 the centrioles; a ''principal piece," which has a thin sheath of questionable 

 origin but no undifferentiated cytoplasm; and an "end piece," which is the 

 naked posterior end of the axial filament. Many minor variations in 

 all of these cell elements are known, the spermatozoa of different animals 

 thus differing widely in appearance (Fig. 135). 



