588 OF REPRODUCTION. 



to the same category with the latter. It may be best studied in those animals 

 which only have a periodical fertility ; and the Passerine Birds are among the 

 most convenient subjects for the purpose. During the winter, the testes are 

 small and almost bloodless, and no trace of Spermatozoa can be detected within 

 them ; on the return of spring, however, they undergo great enlargement and 

 become almost gorged with blood, and the gradual steps of the evolution of the 

 Spermatozoa may be easily observed. The fluid drawn from them is first seen 

 to contain a number of granular corpuscles, resembling those known as the 

 Seminal Granules- in the human semen (delineated at #, Fig. 18, Plate I.) ; 

 and in a short time there are seen, in addition to these, numerous rounded 

 transparent vesicles, at first having but one nucleus, and afterwards presenting 

 several. v These nuclei bear a close resemblance to the granular corpuscles just 

 mentioned ; and it is probable that the former are to be regarded as cytoblasts, 

 from which the Spermatoferous cells (shown, as existing in the human semen, 

 in Fig. 19, Plate I.) are evolved. The nuclei seem afterwards to resolve them- 

 selves into a fine granular matter, which is diffused through the whole vesicle 

 or "cyst of evolution;" and in this, a linear arrangement soon becomes per- 

 ceptible. The lines become more and more distinct, and are at last seen to be 

 evidently produced by the arrangement of the Spermatozoa, which lie side by 

 side within the vesicle ; and the form of this changes from a sphere to a long 

 oval. After a time they break forth, but still adhere to each other for a short 

 period, forming bundles, such as may often be met with in the human semen, 

 when taken directly from the testis (Fig. 20, Plate I.).* That the Spermatozoa 

 are the essential elements of the spermatic fluid, has been reasonably inferred 

 from several circumstances, such as their absence or imperfect development in 

 hybrid animals, which are nearly or entirely sterile : and the fact that Fecun- 

 dation essentially consists in the direct communication of one of them with a 

 certain -point in the Ovum, appears too well established to admit of further 

 doubt. Regarding the uses of the other constituents of the Semen, no suffi- 

 cient account can be given. 



736. The power of procreation does not usually exist in the Human Male, 

 until the age of from 14 to 16 years ; and it may be considered probable that 

 no Spermatozoa are produced until that period, although a fluid is secreted by 

 the testes. At this epoch, which is ordinarily designated as that of Puberty, 

 a considerable change takes place in the bodily constitution : the sexual organs 

 undergo a much-increased development ; various parts of the surface, especially 

 the chin and the pubes, become covered with hair ; the larynx enlarges, and 

 the voice becomes lower in pitch, as well as rougher and more powerful; and 

 new feelings and desires are awakened in the mind. Instances, however, are 

 by no means rare, in which these changes take place at a much earlier period ; 

 the full development of the generative organs, with manifestations of the sexual 

 passion, having been observed in children of but a few years old. The pro- 

 creative power may last, if not abused, during a very prolonged period. 

 Undoubted instances of virility at the age of more than 100 years are on 

 record ; but in these cases, the general bodily vigour was preserve4 in a very 

 remarkable degree. The ordinary rule seems to be, that sexual power is not 

 retained by the male in any considerable degree, after the age of 60 or 65 

 years. To the use of the sexual organs for the continuance of his race, Man 

 is prompted by a powerful Instinctive desire, which he shares with the lower 

 animals. This Instinct, like the others formerly alluded to ( 259 63), is 

 excited by sensations ; and these may either originate in the sexual organs 

 themselves, or may be excited through the organs of special sensation. Thus 



* For a fuller account, with illustrations, of the development of the Spermatozoa, and 

 its analogy with the formation of other tissues, see Princ.of Gen. and Comp.Phys. 430 

 and 607. 



