Ixxxii report — 1877. 



This great disparity of size, however, is connected with an important 

 difference in the disposition of the yolk-substance, according to -which 

 ova may he distinguished as of two kinds — the large- and the small-yolkcd 

 ova, between which there are also many intermediate gradations. The 

 larger-yolked ova belong to the whole tribe of birds, scaly reptiles, osseous 

 and cartilaginous fishes, and the Cephalopods among, the Invertebrates ; and 

 are distinguished by the strictly germinal part or protoplasm being collected 

 into a small disk, known familiarly as the cicatricida 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 deeper yellow colour, contains 

 a large quantity of vitelline 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 germinal disk, and the rest of the yolk serves in a 

 secondary or more remote manner to furnish materials for nourishment of 

 the embryo and its accessory parts. Thus we distinguish the germinal from 

 the nutritive or food-yolk, or, as the younger Yan Beneden has named them, 

 the protoplasm and the deutoplasm. 



In the smaller ovum of the mammal, on the other hand, it seems as if the 

 whole, or nearly the whole, of the yolk were protoplasmic or germinal. 

 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 substance of the yolk, or, in other words, the yolk is entirely 

 germinal. Hence some have given the names of merohlastic and holollastic 

 (meaning partially and entirely germinal) to these two contrasting 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 Am- 

 pMo.vus may be included 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 nutritive or 

 vitelline the other. But among the Invertebrates 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 refe- 

 rence to its first origin, carries us back to a very early period of the develop- 

 ment 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 reproductive elements originally spring. All that I can ven- 

 ture to say at present in regard to this point is, that the primordial ova or 



vol. ii. p. 374, and the Review by Ray Lankester of Haeckel's work, ' Perigenesis dor 

 Blastidule,' &c, in 'Nature' for 1876, p. 235, and Ray Lankester's essay on ' Comparative 

 Longevity,' 1870. 



