282 ANIMAL PHYSIOLOGY. 



into exceedingly complex tubular glands, with their ducts in 

 Structural continuity with them. The technical names of 

 the different parts of the ducts are mentioned in the explana- 

 tion of the accompanying diagram. It is sufficient to note 

 that the length of channel through which the secretion has 

 to pass is without parallel in the rest of the body ; that the 

 secreting tubules are upwards of two feet long, and that the 

 duct called the epididymis is estimated as being twenty feet 

 in length, and is ciliated. The advantage gained by this 

 complexity is not known. The main ducts open into the 

 urethra, a little in front of the bladder, where that passage 

 is surrounded with a glandular structure called the prostate; 

 and in the middle line, close to the two ducts, is a small 

 pouch, just big enough to admit a probe, called the sinus 

 pocularisj interesting as being the structure which, in the 

 female, is developed into uterus and. vagina. 



The male germs, or essential elements of the secretion 

 of the testes, are called spermatozoa. They are bodies vary- 

 ing from -^ (j to ^o- of an inch in length, and consist of a 

 pear-shaped body, with a tail extending 

 out from the broad end. The tail moves 

 with a rapid undulatory movement, which 

 sends the spermatozoon forwards, body 

 foremost. The spermatozoa are developed 

 in the interior of cells, the protoplasm 

 of which has been observed adhering to 

 their heads in the young state, and they 

 may very probably be regarded as ele- 

 ments morphologically equivalent to 

 Fig. 145. SPEBMA- nuclei. Their development is not com- 

 TOZOA. pleted till they leave the secreting tubes. 



206. Observations on the lower animals leave no doubt 

 that fertilization of the ovum takes place by the entrance 

 of spermatozoa into the interior of the zona pellucida, after 

 which, both spermatozoa and germinal vesicle are melted 

 down in the yelk, which thereby acquires new properties, 

 and divides first into two parts, then into four, and so on, 

 each part dividing always into two, until the whole yelk is 

 converted by this process of cleavage into a mass of nucleated 

 corpuscles, devoid of cell walls, from a certain number of 



