The development of the human body; a manual of human embryology . digestive cavity; gr, area of mesoderm formation; mes, mesoderm.—(Eycleshymer.) the primitive streak from that formed from the dorsalwall of the archenteron by speaking of the former as theprostomial and the latter as the gastral mesoderm, thoughit must be understood that the two are continuous imme-diately in front of the definitive blastopore. In the reptilia still greater modifications are found inthe method of formation of the germ layers. Before theenveloping cells have completely surrounded the yolk- 68 THE DEVELOPMENT OF
The development of the human body; a manual of human embryology . digestive cavity; gr, area of mesoderm formation; mes, mesoderm.—(Eycleshymer.) the primitive streak from that formed from the dorsalwall of the archenteron by speaking of the former as theprostomial and the latter as the gastral mesoderm, thoughit must be understood that the two are continuous imme-diately in front of the definitive blastopore. In the reptilia still greater modifications are found inthe method of formation of the germ layers. Before theenveloping cells have completely surrounded the yolk- 68 THE DEVELOPMENT OF THE HUMAN BODY. mass, a crescentic groove, resembling that occurring inamphibia, appears near the posterior edge of the blasto-derm, the cells of which, in front of the groove, arrangethemselves in a superficial layer one cell thick which maybe regarded as the ectoderm (Fig. 26, ec) and a subjacentmass of somewhat scattered cells. Later the lowermostcells of this subjacent mass arrange themselves in a con-tinuous layer constituting what is termed the primary. V Fig. 25.—Section through an Embryo Amphibian (Triton) of 2\Days, showing the Formation of the GasTral Mesoderm. ak, Ectoderm; ch, chorda endoderm; dk, digestive cavity; ik, endo-derm; mk1 andmk2, splanchnic and somatic layers of the meso-derm. D, dorsal and V, ventral.—(Hertwig.) endoderm (en1), while the remaining cells, aggregatedespecially in the region of the crescentic groove, form theprostomial mesoderm (prm). In the region enclosed bythe groove a distinct delimitation of the various layersdoes not occur, and this region forms the primitive groove now begins to deepen, forming an invaginationof secondary endoderm, the extent of this invaginationbeing, however, very different in different species. Inthe gecko (Will) it pushes forward between the ectodermand primary endoderm almost to the anterior edge of the THE FORMATION OF GERM LAYERS. 69 blastoderm, but later the cells forming its floor, together
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
