THE SEMEN. 183 



In man, and in some animals which approach near to man 

 in their organization, the spermatozoa are small, the head or 

 disc is ovate, the narrow extremity forming the summit of 

 the disc, and the tail, proceeding from its broader end, di- 

 minishes in size from its origin to its termination, which is 

 so fine, that its extreme point can with difficulty be discerned. 

 (See Plate XVI. fig. 1.) 



In the rat and in the mouse the seminal animalcules are 

 large, and their form peculiar ; the head is half sagittate, and 

 moveable upon the tail, which is long, and attached not to 

 the base of the arrow-like head, but to its side : frequently 

 the head is curved, in which case it resembles the blade of a 

 curved scimitar. (See Plate XVII.) 



In the guinea-pig also, the magnitude of the spermatozoa 

 is great, and the shape remarkable : in this animal the head is 

 large, ovate, concave on one side, and convex on the other, 

 the tapering and elongated cauda arising from the narrow 

 end of the oviform head, which may be compared in form to 

 a mustard-spoon. (See Plate XVII.) 



In birds two singular types occur, the one characteristic 

 of the order Passeres, the other typical of the Rapaces, 

 Scansores, Gallince, Grallce, and Palmipedes. In the first, 

 the head of the spermatozoon is elongated and spiral, resem- 

 bling a corkscrew ; the number of coils and the acuteness of 

 the angles vary in different species ; usually two, three, or 

 four turns are described ; the attenuated and greatly produced 

 tail proceeds from the finer end of the spire, and between it 

 and the body no exact line of demarcation exists. (See 

 Plate XVI. Jig. 2.) In the second, there is a distinct divi- 

 sion into head and tail, and the former is elongated, as in 

 the order Passeres ; but, in place of being spirally coiled, is 

 straight, the tail arising abruptly from the body, is of great 

 tenuity, of equal diameter, and the length exceeding but little 

 that of the body. 



The various forms assumed by the spermatozoa amongst the 

 other vertebrate and invertebrate animals it is unnecessary 

 here to describe : the shape of those, however, belonging to 

 the Tritons and Salamanders, from their great peculiarity, 



