. Quain's elements of anatomy . Fig. 652.—Section op the babbits blastoderm at six days. (From Beueden.) ed., upper layer, or Eaubers cells ; incs., middle layer; ent., lowerl ajer. E. Van Beneden takes quite a different view of these phenomena (No. 105).Not admitting the transitory natui-e of the upper layer as described by Eauber, heholds that this layer, which is co-extensive with the whole blastodermic vesicle,becomes the epiblast, that from the more restricted plate of deeper cellsthere is first formed a primitive lower layer, and that subsequently there takesplace a separation or d
. Quain's elements of anatomy . Fig. 652.—Section op the babbits blastoderm at six days. (From Beueden.) ed., upper layer, or Eaubers cells ; incs., middle layer; ent., lowerl ajer. E. Van Beneden takes quite a different view of these phenomena (No. 105).Not admitting the transitory natui-e of the upper layer as described by Eauber, heholds that this layer, which is co-extensive with the whole blastodermic vesicle,becomes the epiblast, that from the more restricted plate of deeper cellsthere is first formed a primitive lower layer, and that subsequently there takesplace a separation or differentiation of this into a smaller central intermediateplate of rounded mesoblastic cells and a wider lower layer of fiat hypoblasticcells (see fig. 652). Fig. Fig. 653.—Ovum of amphioxus in the three stages of blastuia, invaginationand gastrula. (From Haeckel, after Kowalevsky.) A. blastuia stage, or single vesicular layer of cells resulting from segmentation ; B, in-vagination stage forming two layers ; C, gastrula stage in which a primitive alimentarycavity is enclosed by a bilaminar blastoderm ; s, segmentation cavity; pet, outer layer ;ent, inner layer; al., primitive alimentary cavity formed by invagination ; h, blastopore. GENERAL EELATIONS OF THE BLASTODERM TO THE OVUM. Invagination of the Blastoderm, Gastrsea Theory.—The brilliant discoveryby Kowalevsky in 1867 (No. 83) of the production of a bilaminar condition of theblastoderm in Amphioxus and some invertebrate animals, by the infolding or in-vagination of a primary simple cellular blastodermic membrane (blastuia) resulting THE BLASTODEEM AND OYUM. 759 from the primary segmentation, and the attempt of Haeckel (No. 89) which fol-lowed to show that a somev/hat analogous process
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