CH. LIX.] FERTILISATION . 881 



by the recent work of O'Donoghue. One must, however, point out 

 that as a rule there is no corpus luteum in the ovaries during 

 menstruation, or during the pre-oestral period in sows, when the 

 mammary glands show signs of growth and congestion. 



The therapeutic use of ovarian extracts appears, from the evidence available, 

 to stand in the same uncertain position as that of testicular extracts. 



If the reader refers back to the chapter on the ductless glands he will find 

 that some of these (thymus, pituitary, etc.) influence the formation of the genera- 

 tive organs. 



The structure of the mammary glands and the composition of milk have 

 already been treated at length in our chapter on Foods (see pp. 481-486). 



FERTILISATION 



"We are now in a postion to resume the thread of the history of 

 the further development of a new individual. The next step is the 

 union of the male and female gametes, that is to say, of the 

 spermatozoon and the mature ovum. 



The act of coitus or copulation is associated in both sexes with 

 much psychical excitement, and with the phenomenon of erection 

 (see p. 313). The spermatozoa are thus deposited at the entrance of 

 the uterus, and by means of the flagellar movement of their tails 

 they make their way against the stream of ciliary movement into 

 the Fallopian tubes, where they are found in a living condition for 

 many days. It is here that they meet the mature ovum; but 

 fertilisation or impregnation only requires the entrance of one 

 spermatozoon into the ovum. By means of its sharp head 

 cap the spermatozoon pierces the 



zona pellucida, and the head, ^ ^~ ZONA PELLUCIDA 



neck, and possibly part of the 

 body, enter the substance of the 

 ovum, where they undergo trans- 

 formation, and are converted 

 into a male pronucleus with an 

 attendant attraction sphere and 

 its centrosome. The male pro- 

 nucleus contains the same number 

 of chromosomes as the female 

 pronucleus, for the mitosis which FIQ 564- _ T1 ^-^ d ova or blastosphere) 



OCCUrS when the primary Sper- showing its new nucleus and attraction 



. . . , __ > spheres ; the yolk granules have been omitted. 



matocyte divides to form two 



secondary spermatocytes, is a heterotype mitosis, in which only 

 half the usual number of chromosomes appear; and consequently 

 the secondary spermatocytes, and their descendants the spermatids, 

 also contain only half the typical number of chromosomes. These 

 are retained in the spermatozoa, which are produced by modifica- 

 tion of the spermatids, and they reappear in the male pronucleus. 



3 * 



