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Amphioxus anteriorly archenteron area pellucida areas opaca avian embryo axial line blasto blastoderm taken blastodisc blastomeres blastopore blastula chalaza chick blastoderm concrescence derm described diameter differentiation disc dorsal lip egg is laid embryo embryo Fig epithelium experimental fore-brain formation formed future embryo gastrulation germ layers germ-ring germ-wall gut-entoderm halves hours after fertilization hours before laying injured cells injured region injury inner edge involution lateral margins lower segmentation cells median line median longitudinal section mesoblast mesoderm needle neural fold numbers opaca and pellucida opaque operating in Experiments ovum pellucid area peristomal Pigeon's Egg Plate portion posterior end posterior margin posterior zone preceding primitive streak material regarded region of overgrowth result section Fig section taken selachian shown in Fig shows side slightly somites stages subgerminal cavity surface views teleostean text for description thin three-fourth hours tion transverse section unincubated vertebrates vitelline membrane Whitman yolk lying beneath zone of junction
Page 122 - The Genesis of Fault-Bars in Feathers and the Cause of Alternation of Light and Dark Fundamental Bars.
Page 95 - A diagrammatic reconstruction of a blastoderm taken thirtysix hours after fertilization, or five hours before laying. It represents the ectoderm as transparent. O, region of overgrowth; Z, zone of junction; Y, germ-wall cells beneath which the subgerminal cavity has spread; D, dorsal lip of the blastopore; PA, outer boundary of the area pellucida ; E, region covered by the invaginated or gut-entoderm.
Page 91 - Entoderm." (p. 439.) In one respect this account is incorrect; for it represents the ectoderm as spreading beneath the entire blastodisc and forming a floor to the cleavage-cavity. There is a plain rolling-under, or involution, as an initiatory step in the formation of the ring ; but we believe that the process is, in the main, more correctly described as an ingrowth, due both to a rapid multiplication of the cells, and also to the centrifugal expansion of the ectoderm.
Page 119 - Nectums in which there is a marked meroblastic tendency, due to the relative increase in the amount of yolk, a lesser extent of the embryo is formed through differentiation in situ, while there is a corresponding increase in the extent of the embryo formed through concrescence, or coalescence of the lateral margins of the blastopore.
Page 95 - The arrows at the posterior margin indicate the direction of movement of the halves of the dorsal lip. X 27-2.
Page 119 - that in those Amphibia which approach most nearly the holoblastic type, as Rana, Bufo, Acris, and Chorophilus, the greater portion of the embryo is formed through differentiation in- situ and overgrowth, concrescence being confined to a limited region at the caudal end of the embryo. In those forms like...
Page 79 - This thinning-out evidently brings about a rapid centrifugal expansion of the disc, for there is no other period in the early history of the blastoderm in which there is such a rapid increase in the surface area, as occurs during the time when the thinning is at its maximum.
Page 68 - ... membrane ; e, wedge-shaped block of yolk containing the blastoderm which is cut out and embedded for sections. plete evidence. So far as I am aware, not a single observer has had a complete series of normal stages of any one type from which to draw his conclusions. The divergent views as to the origin of the entoderm, however, can be grouped into three classes, (i) A number of the older workers have maintained...
Page 119 - Again, in his concluding paragraph he writes that "there is every reason for maintaining that differentiation in situ is the primitive method of embryo formation, concrescence being a secondary process which has progressed pan passu with the increase of yolk material.
Page 68 - ... Concerning the manner in which these two layers arise there has been a wide difference of opinion among embryologists, although a great deal of attention has been paid to this question. The unsatisfactory solution of this problem is due to the fact that most of the conclusions are based on incomFlG.