OF THE TESTES AND SEMINAL FLUIDS. 881 



spontaneously or artificially detached. Among the higher tribes of Animals, 

 as in Man, this mode of reproduction, which is merely a multiplication of 

 the individual, and not a real Generative process, does not present itself, at 

 least in the adult state ; for in no instance do we find that a part of the body 

 separated from the rest can develop the organs which are necessary for the 

 sustenance of its existence ; and the power which the organism possesses, of 

 regenerating parts which it has lost by disease or accident, is restrained 

 within very narrow liaiits ( 361). But there is good ground to believe, 

 that such multiplication by subdivision may take place at that earliest 

 period of embryonic life, at which the germ is nothing else than a mass of 

 cells, wherein no distinction of parts has as yet manifested itself; and that 

 the production of two complete individuals, held together only by a con- 

 necting baud, may arise from some cause which determines the subdivision 

 of the germinal mass, at the period when its grade of development corre- 

 sponds with that of the Hydra or Planaria. And this view of the case is 

 confirmed by the facts already stated ( 361) in regard to the higher degree 

 of the regenerating power during embryonic life, infancy, and childhood, as 

 compared with that which remains after the development of the fabric has 

 been completed. 



729. The proper act of Generation in Man, as in the Animal and Vegeta- 

 ble Kingdoms generally (see PRINC. OF COMP. PHYS., chap, xi), uniformly 

 involves the union of the contents of two peculiar cells, which may be des- 

 ignated as the "sperm cell," and the "germ cell;" and, as in all higher 

 Animals, the " sperm cell " develops in its interior a self-moving spermato- 

 zoon; whilst the "germ cell" (germinal vesicle) whose contents are fertilized 

 by the spermatozoon, is imbedded in a mass of yolk destined for the early 

 nutrition of the embryo thence originating: so that this embryo, if supplied 

 with the requisite warmth, as well as drawing into itself the aliment stored 

 up for it, gradually evolves itself into the likeness of its parent. There is 

 a great difference, however, among the different tribes of Animals, as to the 

 degree of assistance thus afforded to the embryo; the general rule being, 

 that the higher the form which the embryo is ultimately to attain, the longer 

 is it supported by its parent. Hence we find the embryos of most Inverte- 

 brated animals coming forth from the egg in a condition very unlike their 

 perfect type, and only acquiring this after a long succession of subsequent 

 alterations, which frequently involve a complete change of form, or metamor- 

 phosis. In Fishes, however, the embryo, though far from having completed 

 its embryonic development at the time of its emersion from the egg, does 

 not differ so widely from the adult type. In Birds, there is a provision for 

 a much more advanced development; the store of nutritious matter, or 

 "yolk," being so large as to allow the whole series of changes requisite for 

 the formation of the complete chick to take place before it leaves the egg. 

 In the Mammalia, on the contrary, the quantity of yolk contained in the 

 ovum is very small, but the embryo is only dependent upon it for the mate- 

 rials of its increase during the earliest stages of its evolution ; for it speedily 

 forms a special connection with the parent structure, by means of which it 

 is enabled to receive a continual supply of newly prepared aliment, so as to 

 be supported at the expense of this until far advanced in its development. 

 Some approaches to this arrangement are met with among certain of the 

 lower animals, but it is only in the higher Mammalia that it is completely 

 carried out; and it is only in this class, too, that we find a supplemental 

 provision for the nutrition of the offspring after it has come forth into the 

 world. A considerable difference in the ova of different animals further 

 exists, as was first shown by Reichert, in the circumstance that in some the 

 whole yolk undergoes segmentation, and is applied directly to the formation 



