440 OF GENERATION AND DEVELOPMENT. 



the generative apparatus of the parent stock, furnished (it may be) with 

 nutrient and locomotive organs of its own ; and that neither can be re- 

 garded as a complete organism without the other. Thus, the Medusa 

 contains the proper generative apparatus of the Zoophyte, which 

 developes no other; arid the "aggregate" Salpse that are budded- 

 forth from a kind of stalk in the interior of the " solitary" form, must 

 be regarded as altogether constituting its true generative apparatus, 

 since it never produces any other. In all instances it will be found, 

 that whatever may be the variations which present themselves in the 

 entire history of any species, the immediate product of the true Gene- 

 rative act is always the same. 



783. This act, in Animals as in Plants, requires the concurrent 

 action of two sets of organs, evolving "sperm-cells" and "germ-cells" 

 respectively ; and it is curious that these should present the closest 

 approach to those of the higher Cryptogamia, rather than to those of 

 Plants above or below them in the scale. The two sets of organs may 

 be united in the same individual, as they are in most Plants ; and the 

 ova may be fertilized from the seminal cells of the same being ; as 

 happens in many Zoophytes and in some of the lowest tribes of Mol- 

 lusca. Or, the two sets of organs being present in each individual, it 

 may not be capable of self-impregnation ; but, in the congress of two 

 individuals, each impregnates, and is impregnated by, the other; as 

 may be observed in the Snail, and many of the higher Molluscs. Or 

 the sexes may be altogether distinct ; one individual possessing only the 

 male or spermatic organs ; and the other the female, or germ-nourishing 

 apparatus ; this is observed in the higher classes of the Radiated, Mollus- 

 cous, and Articulated sub-kingdoms ; and it is the case in all Vertebrata. 



784. The earliest part of the history of Embryonic Development is 

 nearly the same in all Animals ; for it consists in the multiplication of 

 the single cell of which the original germ is composed, until a cluster is 

 formed, all the cells of which appear to be in all respects similar to one 

 another. Each of these cells either takes into itself, or draws around 

 it, a portion of the vitellus or yolk, which is the nutrient substance of 

 the ovum ; and thus either the whole of this vitellus, or a portion of it, 

 is subdivided into a number of minute spherules, altogether constituting 

 what is known as the "mulberry mass" (Fig. 135). The former seems 

 to be the case, when the grade of development of the organism which is 

 to be formed at the expense of the yolk is very low ; whilst the latter 

 plan is followed, when the yolk is destined to afford a prolonged suste- 

 nance to the embryo, which attains a high degree of development whilst 

 supported upon it alone. Thus among the Invertebrata generally, we 

 find that the embryo comes forth from the egg in a very simple condi- 

 tion, a large part of its structure having undergone but little change 

 from the state of the "mulberry mass;" and in these, the whole yolk 

 undergoes subdivision. The same is the case, too, in the Batrachian 

 Reptiles, which issue from the egg in a form very different from that 

 into which they are to be subsequently developed ; and it is the case 

 even with Mammalia, but for a very different reason, their embryonic 

 structure, formed at the expense of the yolk, being destined to acquire 

 additional material for its full development from a source altogether 



