The development of the human body; a manual of human embryology . Fig. 23.—Transverse Section opAmphioxus Embryo with FiveMesodermic Pouches. Ch, Notochord; d, digestive cavity; THE FORMATION OF THE GERM LAYERS. 6/ As the blastopore diminishes in size and its lips cometogether and unite the ring of mesoderm forms first anoval and then a band lying beneath the line of closure ofthe blastopore and united with both the superjacentectoderm and the subjacent endoderm. This line offusion of the three germ layers is known as the primitivestreak. It is convenient to distinguish the mesoderm of. Fig. 2
The development of the human body; a manual of human embryology . Fig. 23.—Transverse Section opAmphioxus Embryo with FiveMesodermic Pouches. Ch, Notochord; d, digestive cavity; THE FORMATION OF THE GERM LAYERS. 6/ As the blastopore diminishes in size and its lips cometogether and unite the ring of mesoderm forms first anoval and then a band lying beneath the line of closure ofthe blastopore and united with both the superjacentectoderm and the subjacent endoderm. This line offusion of the three germ layers is known as the primitivestreak. It is convenient to distinguish the mesoderm of. Fig. 24.—Section Through a Gastrula of Amblystoma. dli Dorsal lip of blastopore; gc, 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
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
