THE GERMINAL DISC. 953 



development, actually forms the young animal. This is also the case 

 in the Echinodermata, the smaller Crustacea, and Arachnida, the In- 

 secta, and the Mollusca, except the Cephalopods. But in the higher 

 animals, with holoblastic ova, such as the Cyclostomatous Fishes, the 

 Amphibia, and the Mammalia, the cleavage-cells form, at a certain 

 part of the germ-sac, a small opaque hemispherical mass, which soon 

 spreads out into a disc-like layer, and constitutes the so-called germinal 

 disc or area, area germinativa, embryo-spot, or blastoderm. It is the 

 central part of this which is directly concerned in the formation of 

 the embryo. In the Molluscous, and other animals just enumerated, 

 no such germinal area is found. In all the meroblastic ova, whether 

 of Non-vertebrate or Vertebrate type, as is well seen in the hen's egg, 

 the cicatricula, or germinal disc, already described, constitutes this 

 germinal area or embryo-spot. 



In this area, as in that of the higher holoblastic ova, the first traces 

 of the embryo are formed. Amongst the Non-vertebrate types, those of 

 the higher Arachnida, and Crustacea, they appear as a certain number 

 of opaque spots, having a beautiful symmetrical arrangement, whilst 

 in the Cephalopods, they commence by a small number of primitive 

 masses. In the Vertebrate holoblastic or meroblastic ova, the com- 

 mencement of the embryo is always indicated, amongst other things, 

 by the appearance of a linear primitive streak. This is quite charac- 

 teristic of the Vertebrate type of ovum, not occurring even in the 

 elongated Annulose type. The evolution of the Vertebrate embryo 

 can alone occupy our attention here. That of the chick in ovo will be 

 generally followed ; but the peculiarities of the ovum of the Mammalia 

 will also be indicated. 



When first formed, the appearance of the blastoderm, or blasto- 

 dermic layer of the germinal area, is nearly or quite uniform, all its 

 nucleated cells being alike, and the result of a homogeneous evolution. 

 But soon a heterogeneous development ensues, cells of different char- 

 acter, and collected in peculiar situations, appear, and, by more special 

 aggregations, and wider differentiations, the various parts which form 

 the embryo, its organs, tissues, and appendages, are evolved. 



First, the germinal area or disc increases in size or thickness, by 

 the formation of new cells ; as already mentioned, it very early con- 

 sists of two layers, named the upper, external, or serous, and the lower, 

 internal, or mucous germinal lamina, or plate ; between these, a middle 

 germinal layer, lamina, or plate, is soon formed, but rather in connec- 

 tion with the serous layer. The internal layer, epithelial in structure, 

 is soon prolonged over the germinal sac, which covers the yolk ; the 

 outer layer also extends itself, but the middle layer does not pass be- 

 yond the limits of the embryo-spot. As the germinal area enlarges, it 

 presents a central transparent region, known as the transparent area, 

 or area pellucida, around which is a denser portion, named the opaque 

 area, or area opaca ; beyond this is the vitelline area. The trans- 

 parent area is at first circular, but soon oval, and afterwards pear- 

 shaped ; in it, the first rudiments of the embryo appear, in the form 

 of a linear oblong mark, or streak, called the primitive trace, or groove. 

 This consists of a median or axial furrow, bounded by two lateral 



