THE SEXUAL ELEMENTS OVUM AND SPERMATOZOON. 



13 



plasmic or vegetative pole below. An exception to this is found, however, in 

 the pelagic teleost eggs, which float with the deutoplasmic pole upward. 



In the hen's egg the cytoplasm and deutoplasm are distinct and separate 

 with no mingling of the two substances (Fig. 7). While still in the ovary, the 

 egg consists of the yellow yolk in the form of an enormously large cell sur- 

 rounded by the zona pellucida, upon which lies a small white spot, the so- 

 called germinal disk. The disk is 3 or 4 mm. in diameter and consists of 

 finely granular protoplasm with a somewhat flattened nucleus. This disk 



Germinal disk (cytoplasm) 



White yolk 



Albumen ("white 



Vitelline membrane 



White yolk 



Shell membrane 

 (outer layer) 



Chalaza 



Shell membrane 

 (inner layer) 



Yellow yolk (deutoplasm) 

 FIG. 7. Diagram of a vertical section through an unfertilized hen's egg. Bonnet. 



alone gives rise to the embryo proper. All the rest of the mass consisting of a 

 vast number of spherules united by a small amount of cement substance, is 

 simply nutritive material or deutoplasm which is later utilized for the nourish- 

 ment of the embryo. 



THE SPERMATOZOON. 



In marked contrast to the ovum, the spermatozoon is one of the smallest cells 

 of the body, being only about fifty microns in length. The spermatozoon, as 

 seen in the seminal fluid, in any of the sexual passages, or even in the lumen of a 

 seminiferous tubule, is a true sexual element, since it has passed through certain 

 processes which prepare it for union with the mature ovum. (See Spermatogen- 

 esis, Chap. III.) Like the ovum the spermatozoon is an animal cell of which, 

 however, both cell body and nucleus have undergone important modifications. 

 The flagellate spermatozoon, of which the human spermatozoon is an example 

 (Fig. 8), resembles a tadpole in shape and like the latter swims about by 

 means of the undulatory movements of its long slender flagellum or tail. It 

 consists of (i) a head, (2) a middle-piece or body and (3) a tail. 



i. THE HEAD. This in the human spermatozoon is from three to five 

 microns long and about half as broad. On side view it appears oval ; when 



