946 THE OVUM. 



undergoing division and toward the termination of this process still connected 

 with filaments. These are designated seminal cells (/, 5 and //, ///, IV, p}. 



Contrary to the description just given the formation of spermatozoa has 

 recently been described as follows: The spermatozoa originate from seminal 

 cells, spermatogonia or primitive seminal cells. These multiply by indirect 

 division, move toward the center of the seminal tubule, increase in size and are 

 known as spermatocytes or seminal mother-cells. Each one now further sub- 

 divides into four cells the spermatids or seminal cells, which move further 

 toward the lumen. Each of these develops into a seminal filament or spermato- 

 coma, the nucleus of the cell forming the head, a small portion of the cell-pro- 

 toplasm the cilium of the spermatozoon, while the axial fiber of the latter develops 

 from the central body of the cell. For the complete development of the sper- 

 matozoa, it is now necessary that the spermatids (seminal cells) unite with the free 

 extremity of Sertoli's "supporting cells" through a form of copulation; and here 

 mature as upon a nourishing stem and finally become detached. The united 

 formations are the spermatoblasts of Ebner. 



In most animals, the spermatozoa are capillary in form, with larger or smaller 

 heads. The latter are elliptical or pear-shaped (mammals) or cylindrical 

 (birds, amphibia, fish) or spiral (singing birds, sharks, viviparidas) or simply 

 capillary (insects and other animals) (Fig. 354). Nonmotile seminal cells differ- 

 ing entirely from the capillary form are found in myriapods and oysters. 



Considerable interest has been aroused by the subcutaneous injection of 

 orchitic extract recently made by Brown-Sequard. This greatly increases the 

 ability to indulge in muscular exercise ; coincidently with the diminution in fatigue 

 there is a diminution in the subjective sense of fatigue. There is greater endur- 

 ance and recuperation has an increased influence. 



The significance of the secretion of the accessory sexual glands (prostate, 

 seminal vesicles, Cowper's gland) has not been fully made clear. That these play 

 some part in the act of procreation is evident from the fact that after their extir- 

 pation the impregnating power of the semen often ceases. In rats castrated 

 before puberty, the accessory sexual glands do not develop. Hammar found 

 secretion also in the epididymis of dogs. Castration or division of the seminal 

 tubules causes atrophy of the prostate. The secretory nerves of the prostate 

 are the ^ypogastric. The seminal vesicles preserve their function in sexually 

 mature individuals even after castration. 



THE OVUM. 



The human ovum (from 0.18 to 0.2 mm. in diameter) is a globular, 

 cell-like structure, presenting a thick, firm, elastic capsule, with delicate 

 radiating striations (oolemma or zona pellucida), protoplasmic, granular 

 contractile contents (yolk, vitellus), including a clear vesicular nucleus 

 from 40 to 50 /j. in diameter and possessing a nuclear framework (ger- 

 minal vesicle) and a nucleolus from 5 to 7 [j. in diameter endowed 

 with ameboid movement (germinal spot). The chemical composition 

 of the ovum is described on p. 423. 



The zona pellucida (0.02 thick) (Figs. 356, 357), to whose surface cells of 

 the cumulus oophorus often adhere, may be regarded as a cuticular membrane 

 developed secondarily from the follicle. Internally to it in many mammals and 

 directly upon the yolk lies a delicate membrane, which is probably the original 

 cell-membrane of the ovum. Between the zona pellucida and the yolk lies a 

 small perivitelline space (Fig. 356). The finely radiate striations of the zona 

 pellucida are due to the presence of numerous pore-canals, through which the 

 adjacent cells of the granulosa send processes for purposes of nutrition. 



In the ova of many animals holothurians, many fish, for example stickle- 

 backs, mussels, etc. a special micropyle is observed. In addition, some ova 

 possess a number of pore-canals collected together at a special area of the ovular 

 membrane (many insects, for example the flea), and these serve partly as a 

 means of ingress to the spermatozoa and partly for the respiratory interchange 

 of gases. 



The yolk has a peripheral clear layer, which encloses a finely granular layer 

 and the latter finally a central mass containing numerous granules yolk-granules 

 or van Beneden's deutoplasm (Fig. 356). 



