52 HISTORY OF THE HUMAN BODY 



such extremes as those of the elephant and the mouse; in 

 the birds, on the other hand, the true egg, i. e., the yellow 

 sphere usually termed the " yolk," is approximately propor- 

 tionate to the size of the parent. This difference is due to 

 the fact that in mammals the egg is little more than the first 

 cell of the new individual, since the food supply comes en- 

 tirely from outside sources, while in birds the food is placed 

 wholly within the egg and is the only source available to the 

 young bird. 



The spermatozoon, never having yolk to give it bulk, is al- 

 ways small, usually far beyond the limits of the unaided eye. 

 Its form is typically that of an oval cell-body or " head " 

 to which is attached a locomotive flagellum, which may at- 

 tain an appreciable dimension in respect to length, but is al- 

 ways extremely delicate. 



When the seminal fluid and the ova are brought together 

 there is always a vast excess of spermatozoa, and in cases in 

 which direct observation has been possible, as in aquatic forms, 

 in which the mingling of the elements occurs freely in the 

 water, the eggs are seen to be assailed by dozens of active 

 spermatozoa, each endeavoring to effect an entrance. To 

 permit the entrance of one and only one of the entire number, 

 several devices are made use of by the eggs of various species ; 

 one of these is the encasement of the entire ovum in a shell, 

 in which there is a single minute opening, the micropyle, 

 through which a single spermatozoon enters and in so doing 

 effectually blocks the way for all successors. In other cases 

 the entrance of a spermatozoon seems to cause some chemical 

 or physical change which renders the egg substance impervious 

 to the other male cells or incompatible with their continued ex- 

 istence. In the eggs of echinoderms (star-fish, sea-urchins, 

 etc.) the stimulus of an entering spermatozoon causes the im- 

 mediate formation of an external membrane which effectually 

 prevents any farther entrance. In mammals it is probable 

 that the zona radiata proves an impassable barrier to all sper- 

 matozoa except those that approach it in a direction perpen- 

 dicular to its surface, thus greatly reducing the number that 



