REPRODUCTION 



747 



consists of a conoid slightly flattened head, from the posterior part of 

 which there projects a short straight rod, provided with^a long filamentous 

 tail or cilium and an end-piece (Fig. 328). The head contains a nucleus 

 of chromatin material. The total length of a spermatozoon varies from 

 50 to 80 micro-millimeters. The characteristic physiologic feature of 

 spermatozoa is incessant locomotion when in a suitable medium. So long 

 as they are confined to the vas deferens they are quiescent, but with their 

 advent into the vesicula seminalis and dissemination in its contained fluid, 

 they become extremely active and move around with con- 

 siderable rapidity. The power of locomotion depends on 

 the possession of the tail which, by lashing the sur- 

 rounding fluid now in this and now in that direction, 

 propels the head from place to place. The vitality of 

 spermatozoa is such as to enable them to retain their phy- 

 siologic activities in the uterus for more than eight days. 



The development of spermatozoa from testicular cells 

 as observed in lower animals indicates that each cell gives 

 rise to four embryonic forms spermatids which subse- 

 quently develop into adult spermatozoa. In this process 

 the primary nuclear chromatin undergoes a division, so 

 that each spermatozoon receives but a fractional amount 

 representing one-half the number of somatic chromo- 

 somes. The changes by which this condition is brought 

 about are comparable to the changes exhibited by the 

 ovum, and have for their result a reduction in the quan- 

 tity of hereditary substance to be transmitted. 



Fecundation. Fecundation is the union of the 



spermatozoon (the sperm-cell) with the ovum (the germ- 



cell) and takes place in the great majority of instances in SPERMATOZOON^ 



the Fallopian tube. After the introduction of the sper- Front view, 2, side 



. matozoa into the vagina during the act of copulation. Y le ^> * the he ^- 



, i . J , Al T Al k. Head. m. mid- 



they soon begin to pass upward, into and through, the dle p i ece> 



uterine cavity and out into the Fallopian tube, where e. Terminal 



they accumulate in large numbers and retain their vi- 



tality for some days. The migration is effected by the 



propelling power of the filamentous tail and by the action of the cilia of the 



uterus and tubes. 



From observations made on the behavior of the spermatozoa toward 

 the ovum in lower animals, and on the manner in which their union is 

 effected, the inference may be drawn that a similar procedure takes place in 

 mammals. In lower animals the spermatozoa on approaching an ovum 

 take on increased activity, swimming around it in all directions and appar- 

 ently seeking a point of entrance. In fish and molluscs the zona pellucida 

 presents a distinct opening, the micropyle, through which the spermatozoon 

 passes. Inasmuch as the mammalian ovum is devoid of such an opening, 

 the mechanism of entrance of the spermatozoon is not clearly understood. 

 Notwithstanding their enormous numbers it is generally believed that but a 

 single spermatozoon effects an entrance into the ovum. With the accom- 

 plishment of this, however, the spermatozoon loses its mobility, after which 

 the tail disappears. 



. (4/ter 



fila- 



