98 GENERAL PRINCIPLES OF ZOOLOGY. 



The Spermatozoa, the morphological elements of the 

 male sperm, are so small that their finer structure can be 

 studied only with the strongest powers of the microscope 

 (Fig. 34, a and /?). Easiest to recognize in them is the 

 head, which from its variety of form, spherical, oval, sickle- 

 shaped, etc., often renders possible the specific determina- 

 tion of the spermatozoa. The head is the closely com- 

 pacted chromatic part of the nucleus, and hence colors 

 very deeply in staining fluids. On it rests an unstaining 

 second part, the middle piece, probably the achromatic 

 part of the nucleus, and on the latter again the tail, a long 

 flagellum, which makes possible the active motility of the 

 ripe spermatozoon. Cytoplasm is usually present only in 

 extremely slight traces, as a thin layer surrounding the 

 nucleus. 



The spermatozoa of nearly all animals, except the ncma- 

 todes and crustaceans, are constructed according to the 

 above-described plan. In these two groups it is worthy of 

 notice that the spermatozoa are remarkably large and in- 

 capable of motion, and that they enclose a homogeneous 

 strongly refractive body (Fig. 34, k), previously not found, 

 the significance of which is not clear. The spermatozoon 

 of Ascaris (Fig. 34, e) has the form of a sugar-loaf with a 

 broad rounded end, containing the nucleus ; the sperma- 

 tozoon of the crayfish (Fig. 34, y), on the other hand, has 

 the shape of a cake-pan, from whose periphery springs a circle 

 of fine, stiff, and pointed fibres. 



Sensory Epithelium. The modification of the epi- 

 thelium remaining to be spoken of is the sensory epithe- 

 lium. This obtains its special character through the connec- 

 tion which some of its cells, the sensory cells, enter into 

 with the most delicate branches of the ramifying nerves 

 coming from the central nervous system. The sensory 

 cells concerned are commonly to be recognized by their 

 form ; they are long, fine threads in which a varicosity 

 is caused by the inclusion of the nucleus (Fig. 35). Thus 

 the cell-body consists of a peripheral end which has to 

 do with the reception of the sensation, and a proximal 



