A reference handbook of the medical sciences, embracing the entire range of scientific and practical medicine and allied science . erved in the teleosts and selach-ians. According to this theory, the marginal foldof the blastoderm contains the rudiments of the twohalves of the embryo. ^^hiIe the blastoderm growsover the yolk, these two halves are gradually broughttogether, beginning at the cephalic end (Fig. 677).Minot (IS92) defines concrescence as the growingtogether of the two halves of the ectental line to formthe structural of the future embrj-o. The archistome theory (Urmundtheorie
A reference handbook of the medical sciences, embracing the entire range of scientific and practical medicine and allied science . erved in the teleosts and selach-ians. According to this theory, the marginal foldof the blastoderm contains the rudiments of the twohalves of the embryo. ^^hiIe the blastoderm growsover the yolk, these two halves are gradually broughttogether, beginning at the cephalic end (Fig. 677).Minot (IS92) defines concrescence as the growingtogether of the two halves of the ectental line to formthe structural of the future embrj-o. The archistome theory (Urmundtheorie) foundedby O. Hertwig, is very similar to the concrescencetheory, but differs in the following particulars: (1)In emphasizing the character of the blastopore as avestige of the archistome; (2) in distinguisning twoparts of the margin of the blastoderm, the marginalfold representing marginal lips and the zone of over-growth where the process of cleavage is not complete;and (3) in recognizing that the blastopore is not astructure having a position, but that, beginningat the anterior end, it may migrate along the median. Fig. 677.—Diagram to illustrate the theory of Concrescence:Bl, blastoderm; bl, blastopore; a, a, a, successive stages in theclosure of the blastopore: .V, medullarj groove; nr, neural ridge;pr. s, primitive streak; s, thickened edge of blastoderm; Yk, yolk.(From Minot.) line the whole length of the archistomal to this theory the anterior lip of the archis-tome gives rise to onlj- the anterior part of the headof the embryo, and the rest of the structuresare formed from the lateral margins of the archis-tome, which unite in the mid-line. The anus marksthe posterior end of the archistome. Hubreeht holds similar views as to the relation ofthe archistome and structures, but he laysespecial stress upon the idea of gastrulation in twophases, and differs widely from Hertwig as to the ori-gin of the mesoderm and in other details. To
Size: 1588px × 1572px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorbuckalbe, bookcentury1900, bookdecade1910, bookyear1913
