r 4 2 THE HUMAN SPECIES 



of the ovum, wherein, however, the nucleus ovi (blastodermic 

 vesicle) and the nucleolus (germinal spot) may be distinguished. 

 The diameter of the Graafian follicle is : 



In man ...... 10-12 mm. 



other mammals as much as i cm. and more. 



The mature human ovum measures OT-O'3 mm. 



animal 1-1-5 



These figures show that the human ovum is easily distinguish- 

 able from animal ova. 



A general survey of the entire vertebrate kingdom, including 

 the most highly developed mammals and man, reveals extremely 

 primitive conditions as regards the sexual organs of the lower 

 forms of life. Thus in the Cyclostoma the testicles unite and 

 form a simple organ which, in the absence of excretory ducts, 

 discharges its contents when mature into the abdominal cavity, 

 whence the semen is eventually evacuated. The Selache, Chim- 

 aera and Dipnoi have paired but small testicles with epididymis 

 and on either side a vas deferens, and paired, symmetrical 

 ovaries which, however, are not connected with the oviducts in 

 spite of these being also paired. From the fact that each ovi- 

 duct becomes dilated at its lower extremity there results, on 

 the right as on the left, a kind of uterus, each of which opens 

 into the cloaca. The Ganoidei appear extremely backward in 

 their development, being so simply organised that the ovaries 

 as well as the testicles discharge their products immediately into 

 the abdominal cavity, without the aid of special excretory ducts, 

 and thence through a short canal into the ureter ; in certain 

 species the ureter can open into the above-mentioned canal 

 (Mullerian duct). The highest fishes are the Teleostei 

 with their usually elongated testicles and ovaries, the latter in 

 many species even serving as brood cavity for the development 

 of the embryo. The amphibians in many respects resemble 

 the fishes, the convoluted oviducts being situated beside the 

 paired ovaries and, after uniting with the permanent and active 

 Wolffian ducts, open into the cloaca. On the other hand, the 

 testicles are united with the Wolfifian bodies, one part secreting 

 the semen and another part performing the function of kidney. 

 The Wolffian bodies do not persist in the Vertebrates higher 

 than the reptiles. The male sexual organs consist of testicles 



