690 OF REPRODUCTION. 



902. The mode of Evolution of Spermatozoa, which has been recently 

 discovered by Wagner, is so different from the ordinary method of produc- 

 tion amongst Animalcules, as of itself to indicate that the former cannot be 

 referred 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. 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 themselves into a fine granular matter, which is 

 diffused through the whole vesicle or "cyst of evolution;" and in this, a linear 

 arrangement soon becomes perceptible. 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 Fecundation essentially consists in the 

 direct communication of one of them with a certain point in the Ovum, ap- 

 pears too well established to admit of further doubt. Regarding the uses of 

 the other constituents of the Semen, no sufficient account can be given. 



903. 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 procreative 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 preserved 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 

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



* 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 



und 007. 



