. The elements of embryology . Embryology. 54 THE FIEST DAY. [chap. Sections at this stage throw a very important light on the nature and mode of origin of the primitive streak. In the region in front of it the blastoderm is still formed of two layers only, but in the region of the streak itself the structure of the blastoderm is greatly altered. The most important features in it are repre- sented in Fig. 15. This figure shews that the median Fig. Transverse Section through a Blastoderm of abodt the age represented in fig. 14, shewing the flhst dif- FERENTIATION OP THE Primitive Streak. T
. The elements of embryology . Embryology. 54 THE FIEST DAY. [chap. Sections at this stage throw a very important light on the nature and mode of origin of the primitive streak. In the region in front of it the blastoderm is still formed of two layers only, but in the region of the streak itself the structure of the blastoderm is greatly altered. The most important features in it are repre- sented in Fig. 15. This figure shews that the median Fig. Transverse Section through a Blastoderm of abodt the age represented in fig. 14, shewing the flhst dif- FERENTIATION OP THE Primitive Streak. The section passes through about the middle of the primitive streak. pvs. primitive streak ; ep. epiblast; hy. hypoblast ; 7/k. yolk of the germinal wall. portion of the blastoderm has become very much thick- ened (thus producing the opacity of the primitive streak), and that this thickening is caused by a proliferation of rounded cells from the epiblast. In the very young primitive streak, of which Fig. 15 is a section, the rounded cells are still continuous throughout with the epiblast, but they form nevertheless the rudiment of the greater part of a sheet of mesoblast, which will soon arise in this Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Foster, M. (Michael), Sir, 1836-1907; Balfour, Francis M. (Francis Maitland), 1851-1882; Sedgwick, Adam, 1854-1913; Heape, Walter, 1855-1929. London ; New York : Macmillan
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Keywords: ., bookcentury1800, bookdecade1880, bookpubli, booksubjectembryology
