UROGENITAL SYSTEM 



225 



Resembling in this respect the ova of mammals (and in- 

 deed the majority of the worms, molluscs, echinoderms and 

 polyps), the human ovum possesses in addition to the forma- 

 tive yolk no nutritive yolk, such as exists in birds, reptiles, 

 amphibians and fishes. 



The spermatozoa (Waldeyer's spermia) are the secretion of 

 the male generative glands, and consist of minute, threadlike 

 (caudate), flagellated cells, of which a Single drop of seminal 

 fluid contains millions (Fig. 114). The form is constant in each 

 species, but in the various- orders of animals there are distinct 

 differences in the shape of the so-called head, and in the 



FIG. 1 14. Spermatozoa or spermia of various mammals. The pear-shaped flattened 

 nucleus is seen from the front in I. ; sideways in II. M, human spermatozoa ; 

 A, from the ape; K, from the rabbit; H, from the mouse; C, from the dog; 

 S, from the pig. 



length of tail. It is too great a task to discuss in detail all the 

 variations of every class and order. 



In mammals generally they conform to a single type with 

 numerous modifications, namely, a thick almost disc-shaped 

 head and a flagellated appendage (a threadlike tail). In the 

 ape, man's nearest ally, the head is ovoid, the tail being 

 attached to the blunt extremity. 



In man the spermatozoa have likewise an oval head with 

 the side from which the tail springs being thick and rounded. 

 The head is T? nnj- mm. long, T i/W mm - broad, and - mm. 

 thick ; the tail is T f g^ mm. long. 



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