. Quain's elements of anatomy . rough half the blastodermOP A NEWLY-LAID EGG. (From Strieker.) -2 S, upper layer of small nucleated cells ; D, lo\rer layer of larger granular cells; M, seg-ment spherules lyiug in tiie subgerminal cavity ; A, substance of the white yolk below thegerm. mass of cells, formed by direct proliferation from the lower cells of theepiblast, and continuing to adhere to that layer along the whole lengthof the streak. This is the axial plate of His and KoUiker, and, as firstsho\^n by the latter observer, is undoubtedly the commencement of amiddle layer developed in connec
. Quain's elements of anatomy . rough half the blastodermOP A NEWLY-LAID EGG. (From Strieker.) -2 S, upper layer of small nucleated cells ; D, lo\rer layer of larger granular cells; M, seg-ment spherules lyiug in tiie subgerminal cavity ; A, substance of the white yolk below thegerm. mass of cells, formed by direct proliferation from the lower cells of theepiblast, and continuing to adhere to that layer along the whole lengthof the streak. This is the axial plate of His and KoUiker, and, as firstsho\^n by the latter observer, is undoubtedly the commencement of amiddle layer developed in connection with the epiblast. At this time, that is, after the formation of the primitive streak andprevious to the appearance of the medullary plates and chorda dorsalis,the blastoderm in the forepart of the germinal area, both of birds andmammals, consists of only two layers, and it is not till some time laterthat the mesoblast which gives rise to the protovertebral plates is foundto have extended itself into this region. Fig. Fig. 643.—Transverse section through the front end op the primitive streak andBLASTODERM OF THE CHICK. (From Balfour.) 2or, primitive groove ; m, mesoblast; ep, epiblast; 7iy, hypoblast. With regard to the much debated question of the mode of origin andextension of the mesoblast two different views mainly prevail among theembryologists who have most recently investigated the subject; accordingto one of which, maintained by KoUiker (Nos. 28 and 99), Braun (No. 110)and others, the mesoblast is entirely derived from the axial plate of the Vf) II. 3 c ro4 EELATIOX OF BLASTODERM TO DEYELOPMEXT. epiblast before referred to, and spreads from that source outwards into allthe other parts of the embryo or ovum Avhere it afterwards forms thefoundation of new parts. According to the other vieAv, held by Balfour(Nos. 32 and 96), His (No. Ill) and others, while it is admitted that themcsoblast has the axial origin from epiblast before mentioned, it is main-taine
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