1004 A MANUAL OF PHYSIOLOGY 



undergone a certain amount of differentiation, especially in the 

 higher animals. The fertilized ovum, on the other hand, has the 

 power of reproducing not only ova like itself, but the counterparts 

 of every cell in the body. And this is only the highest development 

 of a power which is in a smaller degree inherent in other cells in 

 lower forms. Plants and the lowest animals are far less dependent 

 upon reproduction by means of special cells. A piece of a Hydra 

 separated off artificially or by simple fission becomes a complete 

 Hydra, as was shown by Trembley a century and a half ago. A 

 cutting from a branch, a root, a tuber, or even a leaf of a plant, 

 may reproduce the whole plant. It is as if each cell in these lowly 

 forms carried within it the plan of the complete organism, from which 

 it built up the perfect plant or animal. 



Reproduction in the Higher Animals. In all the higher animals 

 reproduction is sexual, and the sexes are separate. 



In regard to the secretions of the reproductive glands, all that 

 is necessary to be said here is that, unlike other secretions, their 

 essential constituents are living cells. The spermatozoa in the 

 male have, indeed, diverged far from the primitive type. Certain 

 cells (spermocytes) in the tubules of the testicle divide, each forming 

 two daughter spermocytes. Each of the daughter spermocytes in 

 turn divides, so that four cells (spermatids) are formed from each 

 spermocyte. In the final division which produces the spermatids a 

 reduction of the chromosomes (p. 5) occurs, so that the spermatid 

 possesses only one-half the number characteristic of the somatic cells 

 of the species. The spermatids elongate and become spermatozoa, the 

 head of the latter representing the nucleus of the former ; and it is 

 this nucleus (with the middle piece originally containing the male 

 centrosome and attraction sphere) which is the essential contribution 

 of the male to the reproductive process. The tail of the spermatozoon 

 is simply, from the physiological point of view, a motile arrangement, 

 whose function it is to carry the nucleus of the male element, freighted 

 with all that the father can transmit to the offspring, into the neigh- 

 bourhood of the female reproductive element or ovum. After the 

 spermatozoon has penetrated the ovum its tail disappears, being 

 probably absorbed. 



The ovum also begins as a typical cell with nucleus (germinal 

 vesicle) , nucleolus (germinal spot), centrosome and attraction sphere 

 (p. 5), and it forms, by its repeated subdivision, all the cells of the 

 fretal body. But, except in some (partheno genetic) forms, it never 

 awakens to this reproductive activity till fecundation has occurred ; 

 and fecundation essentially consists in the union of the male with the 

 female element, or rather in the union of the male and female nucleus. 



From time to time a Graafian follicle, overdistended by its liquor 

 folliculi, bursts on the surface of the ovary and discharges an ovum. 

 It was formerly believed that the frayed or fimbriated end of the 

 Fallopian tube, rising up finger-like from the dilatation of its blood- 

 vessels, grasps the ovum. But it is more than doubtful whether this 

 occurs. It is more probable that the ovum is first discharged into 

 the pelvic cavity, and is guided to the orifice of the Fallopian tube, 

 not necessarily that of its own side, by the movements of the cilia 

 around the orifice, and then passed slowly along the tube by the 

 downward lashing cilia which line it. If not impregnated, it soon 

 perishes amid the secretions of the uterus how soon has been 

 matter of discussion, and can hardly be considered as settled. If, 

 however, impregnation occurs, the ovum penetrating the superficial 



