GENERAL HISTOLOGY 



73 



an unstaining second part, the middle piece (4), and then the tail (5), 

 a long flagellum, which causes the active motility of the ripe spermatozoon. 

 Cytoplasm is present only in an extremely thin layer surrounding the 

 nucleus. A centrosome (3) is nearly always present in the middle piece. 



With the exceptions of the Crustacea, nematodes and many myriapods, the 

 spermatozoa are usually constructed after this type, often, with complicated 

 modifications. In the groups just mentioned the spermatozoa are large and 

 immobile and contain a homogeneous body (b) which is strongly refractive; 

 its functions are uncertain. The spermatozoa of Ascaris (V) are shaped like a 

 sugar loaf, the broad, rounded end containing the nucleus. The spermatozoa 

 of the decapod Crustacea (III, IV) have three or more stiff processes arising 

 from the periphery of the cake-like or cylindrical body which contains the 

 refractile body, and in this again a rod (III, i), possibly to be compared to the 

 perforatorium. In other Crustacea the spermatozoa are threads, often of extreme 

 length (7 mm. in many ostracoda). It is noticeable that in some animals there 

 are dimorphic spermatozoa. In Paludina vivipara (the same is true of other 

 Prosobranchs), there arise in the same individual hair- formed spermatozoa with 

 cork-screw shaped heads (Ila) and others, worm-like and with a bundle of 

 flagella at the hinder end (lib). The first of these contains the normal chro- 

 matin mass (eupyreme spermatozoa); the others have very little chromatin 

 (oligopyreme spermatozoa). In many spiders where a similar dimorphism 

 occurs, the second type of spermatozoa is chromatin free (apyremc}. The 

 supposition that the dimorphism of the 

 spermatozoa is connected with sex determi- 

 nation receives support in the study of the 

 spermatozoa of some Hemiptera. Here half 

 of the spermatozoa have one chromosome 

 ('accessory chromosome') more than the 

 other half. Eggs which are fertilized by the 

 relatively oligopyreme spermatozoon proba- 

 bly produce male animals. 



,1 



Sensory Epithelium. The last 

 modification of epithelium is sensory 

 epithelium, characterized by the connec- 

 tion of certain of its cells, the sensory 

 cells, with twigs of branching nerves 

 which arise in the central nervous system. 

 This connection may be of two kinds. 

 In the first the cell is slender and filiform, 

 the position of the nucleus being indicated 

 by a swelling. The peripheral end is concerned with the reception of 

 sensory stimuli, while the deeper end is continued directly into the nerve 

 ends and correspondingly is branched into two or more extremely fine 

 processes which take on the character of nerve fibrillae (fig. 38). In the 

 second type the sensory nerve ends in a ganglion cell beneath the epithe- 

 lium, sending processes into the latter, the ends of these being applied to 



FIG. 38. Sensory epithelium. . 1 , 

 of an Actinian; B, from the olfactory 

 epithelium of man; d, supporting 

 cells; s, sensory cells. 



