. Cunningham's Text-book of anatomy. Anatomy. j .Ectoderm of amnion Allantoic diverticulum 1 of entoderm vesicle â â¢Body stalk mesoderm -â¢-.Extra-embryonic ccelom .Entoderm ^ Mesoderm covering of ' "entoderm vesicle eurenteric canal lty of entodermal vesicle Fig. 32.âSchema of Sagittal Section of Zygote along Line A in Fig. 31. Plasmodial trophoblast Cellular trophoblast Neural groove Mesoderm lining of trophoblast Amnion cavity Extra-embryonic ccelom , Mesoderm of amnion Ectoderm of amnion Mesoderm covering entoderm Entoderm 43 Cavity of entodermal ^J vesicle The Formation of the Notoc
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. Cunningham's Text-book of anatomy. Anatomy. j .Ectoderm of amnion Allantoic diverticulum 1 of entoderm vesicle â â¢Body stalk mesoderm -â¢-.Extra-embryonic ccelom .Entoderm ^ Mesoderm covering of ' "entoderm vesicle eurenteric canal lty of entodermal vesicle Fig. 32.âSchema of Sagittal Section of Zygote along Line A in Fig. 31. Plasmodial trophoblast Cellular trophoblast Neural groove Mesoderm lining of trophoblast Amnion cavity Extra-embryonic ccelom , Mesoderm of amnion Ectoderm of amnion Mesoderm covering entoderm Entoderm 43 Cavity of entodermal ^J vesicle The Formation of the Notochord and the Secondary Mesoderm.âThe notochord and the secondary mesoderm are formed from the primitive streak; the notochord from its an- terior extremity and the second- ary mesoderm from its lateral margins and posterior end. As soon as the primitive streak is established its anterior end becomes a node or centre of growth by means of which the length and, to a certain extent, the breadth of the body are increased. The portions of the body formed by the activity of the anterior end of the streak are (1) the dorsal portion, from the back part of the roof of the nose, anteriorly, to the posterior end of the trunk, (2) the peri- neum, (3) pelvis, and (4) the ventral wall of the body from I the perineum to the umbilicus. Nevertheless, the primitive streak undergoes little or no in- crease in length; indeed, as growth continues, it becomes relatively shorter as contrasted with the total length of the embryonic region, for the new material, formed by its borders and its anterior extremity, is transformed into the tissues of embryo as rapidly as it is created. The Notochord.âThe notochord or primitive skeletal axis is formed by the prolifera- tion of cells from the anterior end of the primitive streak. On its first appearance it is a narrow process of cells, the head process, which projects forwards from the anterior boundary of the neurenteric canal, between the ectoderm a