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Amphioxus anteriorly archenteron area pellucida areas opaca avian embryo axial line blasto blastoderm taken blastodisc blastomeres blastopore blastula blastulation chalaza chick blastoderm concrescence degenerating derm described diameter differentiation disc dorsal lip egg is laid embryo Fig embryo shown entoderm epithelium experimental fore-brain formation formed future embryo Gastrulation stages germ layers germ-ring germ-wall gut-entoderm halves hours after fertilization hours before laying injured cells injured region injury inner edge invagination involution lateral margins median line median longitudinal section mesoblast mesoderm needle neural fold numbers opaca and pellucida opaque operation in Experiment ovum pellucid area periblastic nuclei periblastlc peristomal Pigeon's Egg Plate portion posterior end posterior margin posterior zone preceding regarded region of overgrowth result section Fig selachian shown in Fig shows side slightly somites subgerminal cavity surface views text for description thin three-fourth hours tion transverse section unincubated vertebrates VIII vitelline membrane Whitman yolk lying beneath zone of junction
Page 95 - RECONSTRUCTION OF BLASTODERM OF PIGEON'S EGG, THIRTY-SIX 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...
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 Necturus in which there is a marked meroblastic tendency, due to the relative increase in the amount of yolk, a...
Page 91 - Entoderm." (p. 439.) In one respect this account is incorrect ; for it represents the entoderm 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 95 - The arrows at the posterior margin indicate the direction of movement of the halves of the dorsal lip. X 27-2under surface of the differentiating ectoderm.
Page 121 - 04. The Fertilization and Early Development of the Pigeon's Egg. The American Journal of Anatomy, Vol.
Page 67 - Breeders have long known that this bird ordinarily lays two eggs at a sitting, the first usually between four and six PM, and the second between one and two PM, on the second day following.
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 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.