REPRODUCTION. 695 



them to retain their physiologic 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 subsequently 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 

 chromosomes. 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 fila- 

 mentous 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. 



The germ nucleus proceeds to the middle of the ovum where it is followed 

 by head and middle-piece of the spermium; the middle-piece forms a central 

 spindle while the germ nucleus and head of the spermium each resolves 

 itself into one-half the number of chromatic loops of a somatic cell. In this 

 condition the fertilized ovum represents a parent cell, that possesses the 

 physiologic activities and characters of both ancestral cells. From this 

 parent cell the offspring develops through successive division, multiplication 

 and differentiation of the resulting cells. The chromatic material of the 

 germ nucleus and head of the spermium represent the transmitters of in- 

 herited characters. 



The Fixation of the Ovum. The ovum, after fertilization in the 

 oviduct, continues to divide and pass slowly to the uterus (8 to 10 days) 

 where it is retained until the end of gestation. A menstrual mucosa having 

 developed the ovum lodges on a smooth thick area and gradually sinks 

 beneath the surface. During the passage down the oviduct the zona pellu- 

 cida has become attenuated and has been finally replaced by a thick layer 

 of ameboid and phagocytic cells called the trophoderm. Upon lodgment 

 of the ovum these cells destroy the underlying mucosa and produce a cavity 



