DEVELOPMENT OF TEE FOEMS OF ANIMAL LIFE. 



4S9 



any doubts as to the presence of a sufficient 

 amount of germinal substance for explaining in 

 the most materialistic fashion the transmission 

 of the organic and other properties and resem. 

 blances between the parent and offspring. For 

 we are led to believe, by those who have recently 

 given their attention to the size of molecules com- 

 posing both living and dead matter, that in such 

 a body as this minute ovum of the mammal there 

 maybe as many as five thousand billions of mole- 

 cules, and even if we restrict ourselves to the 

 smaller germinal vesicle, and, indeed, to the 

 smallest germinal particle which might be made 

 visible by the highest microscopic enlargement, 

 there are still sufficient molecules for all the re- 

 quirements of the most exacting material biolo- 

 gist. 1 



This great disparity of size is, however, con- 

 nected with an important difference in the dispo- 

 sition of the yolk-substance, according to which 

 ova may be distinguished as of two kinds — the 

 large- and the small-yolked ova, between which 

 there are also many intermediate gradations. 

 The larger-yoked ova belong to the whole tribe 

 of birds, scaly reptiles, osseous and cartilaginous 

 fishes, and the cephalopoda among the inverte- 

 brates ; and are distinguished by the strictly ger- 

 minal part or protoplasm being collected into a 

 small disk, known familiarly as the cicatricula of 

 the fowl's egg, and to be seen as a whitish spot 

 on that side of the yolk which naturally floats 

 uppermost ; while the rest of the yolk, of a deep- 

 er-yellow color, contains a large quantity of vitel- 

 line granules or globules of a different chemical 

 nature from the protoplasm; 



The phenomena of embryonic development are, 

 in the first instance at least, confined to the ger- 

 minal disk, and the rest of the yolk serves in a 

 secondary or more remote manner to furnish ma- 

 terials for nourishment of the embryo and its 

 accessory parts. Thus we distinguish the germi- 

 nal from the nutritive or food yolk, or, as the 

 younger Van Beneden has named them, the pro- 

 toplasm and the deutoplasm. 



In the smaller ovum of the mammal, on the 



1 According to a calculation made by Mr. Sorby, the 

 number of molecules in the germinal vesicle of the mam- 

 malian ovum is such that, if one molecule were to be lost 

 in every second of time, the whole would not be exhausted 

 in seventeen years. (See Address to the Microscopic So- 

 ciety in Journal of Microscopical Science, vol. xv., p. 225, 

 and Nature, vol. xiii., p. 332. See also Darwin on Pan- 

 genesis, in his work on " Variations," etc. (1S68), vol. ii., 

 p. 374, and the review by Ray Lankester of Haeckefs 

 work, " Perigenesis dor Blastidule," etc., in Nature for 

 1876, p. 235, and Pay Lankester's essay on " Compara- 

 tive Longevity," 1S70.) 



other hand, it seems as if the whole, or nearly 

 the whole, of the yolk were protoplasmic <>r ger- 

 minal. There may be some admixture of yolk- 

 granules ; but there is not the marked separation 

 or limitation of the protoplasmic-substance which 

 is so distinct in birds, and the earliest changes of 

 development extend to the whole component sub- 

 stance of the yolk, or, in other words, the yolk 

 is entirely germinal. Hence some have given 

 the names of meroblastic and holoblastic, meanins 

 partially and entirely germinal, to these two con- 

 trasting forms of ova. There are many of the 

 invertebrate animals of which the ova present 

 the same entirely germinal arrangement as in 

 those of mammals, and the ampMoxus may be in- 

 cluded in the same group. 



The amphibia stand in some measure between 

 the two extremes ; the purely protoplasmic or 

 germinal part occupying one side, and the nutri- 

 tive or vitelline the other. But among the inver- 

 tebrates the gradations are often such as to make 

 it difficult to determine under which group the 

 ova should be placed. 



The genesis or formation of the ovum itself, if 

 it be considered with reference to its first origin, 

 carries us back to a very early period of the for- 

 mation of the parent in which it is produced ; 

 and it is one of the most interesting problems to 

 determine what is the source of the cells in the 

 parent from which the ova originally spring. All 

 that I can venture to say at present in regard to 

 this point is, that the primordial ova or germs 

 appear in the parental body while still embryonic, 

 at a very early period of its development, and 

 clearly derive their origin from a deeply-seated 

 part of the formative cells which are undergoing 

 transformation into the primitive organs ; but the 

 exact seat of the origin of the reproductive cells 

 is still a matter of doubt. 



When the ovum attains its full maturity in 

 the ovary, the seat of its formation within the 

 parent, it is separated from that organ, and when 

 perfected proceeds to undergo embryonic devel- 

 opment; a marked difference in this respect ex- 

 isting between the germinal product of the higher 

 plants and animals. 



The period of maturation of the ovum is 

 marked in the greater number of animals by a 

 series of phenomena which have generally been 

 interpreted as the extrusion or absorption of the 

 grminal vesicle ; and various observers have 

 actually traced the steps of the process by which 

 that vesicle appears to leave the yolk and is lost 

 to sight, or has passed into the space between 

 the yolk and its membrane in the shape of the 



