26 Journal of Agriculture. [i i Jan., 1909. 



THE ELEMENTS OF ANIMAL PHYSIOLOGY. 



W. A. Osbonie, M .B., D.Sc, Professor of PJiysiology and Histology, Dean 

 of the Faculty of Agriculture in the Vniversity of Melbourne. 



{Continued from -page 721, Vol. VI.). 



XVII.— Reproduction. 



In all vertebrates, except the degenerate sea-squirts, reproduction takes 

 place sexually, that is to say each individual arises by the union of a cell 

 called a sperm cell or spermatozoon derived from the male, with a cell 

 called an ovum derived from the female. In Chapter II. it was shown 

 that every higher animal and plant is built up of cells each of which con- 

 tains a nucleus. Now in every nucleus there are present a number of 

 structures like short pieces of microscopic twine called chromosomes (see 

 Fig. 9). The number of chromosomes in each body cell is constant for 

 the members of a particular species. Thus in some snails there are 32 ; 

 in the mouse, trout, and lily there are 24 ; in the ox, guinea-pig, man, and 

 the onion there are 16. Now it is found that in the spermatozoon and 

 also in the ovum the number of chromosomes is exactly one half that in 

 each of the body cells ; when therefore these two cells fuse together in 

 what is called the fertilization of the ovum the correct number of chromo- 

 somes — and therefore an efficient cell — is produced. There is some reason 

 to belie\'e that the chromosomes are these structures w hich are responsible 

 for the transmission of ancestral characters to the offspring. 



The spermatozoa arise in two testes or testicles. Each spermatozoon 

 is composed of a head, a neck and a tail, the last mentioned part keeping 

 up a constant side to side or wriggling movement, sO' that the spermatozoon 

 can move spontaneously and is thus able to travel several inches or even a 

 few feet on a moist surface before pvhausticn sets in. The spermatozoa 

 are microscopic in size, being about '^\^ inch long for most of the larger 

 mammals. The spermatozoa floating ni an albuminous fluid pass from 

 each testicle through a highly convoluted tube and then through a straight 

 tube close to the neck of the bladder where in most mammals a seminal 

 VESICLE or reser\'oir is found. In the sexual act the secretion of the 

 testicle is mixed with the secretion of other glands such as the prostate. 

 and the mixed fluid or semen is ejaculated through the urethra to the end 

 of the penis and thence into the vagina or uterus of the female. The 

 number of spermatozoa injected in a single insemination is very large 

 being expres.sed as hundreds of millions for most mammals. When the semen 

 arrives in the vagma or uterus the powerful tail movement of each sper- 

 matozoon urges it forward, the direction heing always the same and probably 

 determined b\' a chemical substance which is present in the moisture of the 

 genital passages of the female and which increases in strength from without 

 inwards. What may now be described as a race takes place. The sper- 

 matozoa travel through the uterus and up the l^'allo; ian tutes attached to 

 the uterus. Should an ovum be present in the upijer part of a Fallo- 

 pian tube, or even at the beginning of this tute in" the abdominal cavity, 

 the first spermatozoon to arrive enters it and fertilizes it aiid immediatelv 

 the ovum alters its outer layer or tunic so that no more spermatozoa can 

 enter. (Fig. 62.) There is ];robably here a provision of nature for 

 selecting the most vigorous spermatozoon in somewhat the same way that 

 the queen bee selects the most \igorous drone during her nuptial flight. 



