658 



TEXT-BOOK OF PHYSIOLOGY. 



m 



f 



The development of spermatozoa from testicular cells as observed 

 in lower animals indicates that each cell gives rise to four embryonic 

 forms spermatids which subsequently develop into adult sperma- 

 tozoa. In this process the primary nuclear chromatin undergoes a 

 division, so that each spermatozoon receives but a fractional amount. 

 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 quantity 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 the Fallopian tube. 

 After the introduction of the spermatozoa into 

 the vagina during the act of copulation, they 

 soon begin to pass upward, into and through, 

 the uterine cavity and out into the Fallopian tube, 

 where they accumulate in large numbers and 

 retain their vitality for some days. The migration 

 is effected by the propelling power of the 

 filamentous tail. 



From observations made on the behavior of 

 the spermatozoa toward the ovum in lower 

 animals, and on the manner by 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 apparently seeking 

 a point of entrance. In fish and molluscs the 

 zona pellucida presents a distinct opening, the 

 micro pyle, 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 gener- 

 ally believed that but a single spermatozoon effects an entrance 

 into the ovum. With the accomplishment of this, however, the 

 spermatozoon loses its vitality, after which the body and tail dis- 

 appear. The head, which in this instance also is the transmitter of 

 the inherited material, advances to meet and unite with the nucleus 

 of the ovum. A series of histologic changes now arise, which 

 eventuate in the production of a new cell, a parent cell, possessing 

 all the features of cell structure and the physiologic activities and 



V 



FIG. 323. HUMAN 

 SPERMATOZOON. 



1 . Front view, 



2, side view, of 

 the head. k. 

 Head. m. mid- 

 dle piece. /. 

 Tail. e. Termi- 

 nal filament. 

 (After Retzius.} 



