. The biology of the frog . B FIG. 25. —Sagittal sections through two embryos. In A the blastopore isoverarched and there is the beginning of the proctodeum or anal invagi-nation. In B the proctodeum has met and fused with an evagination ofthe archenteron. A, anus ; FB, forebrain ; HB, hindbrain ; LV, liverdiverticulum; MB, midbrain ; N, notochord; NT, neurenteric canal;PD, proctodeum ; PH, pharynx; PN, pineal body; FT, pituitary body.(From Morgan, after Marshall.) 105 io6 THE BIOLOGY OF THE FROG CHAP. The lining of the mouth cavity is formed from an invagin-ation of ectoderm, the stomodeum, w
. The biology of the frog . B FIG. 25. —Sagittal sections through two embryos. In A the blastopore isoverarched and there is the beginning of the proctodeum or anal invagi-nation. In B the proctodeum has met and fused with an evagination ofthe archenteron. A, anus ; FB, forebrain ; HB, hindbrain ; LV, liverdiverticulum; MB, midbrain ; N, notochord; NT, neurenteric canal;PD, proctodeum ; PH, pharynx; PN, pineal body; FT, pituitary body.(From Morgan, after Marshall.) 105 io6 THE BIOLOGY OF THE FROG CHAP. The lining of the mouth cavity is formed from an invagin-ation of ectoderm, the stomodeum, which pushes in until itbreaks through into the archenteron. A similar ectodermalinvagination, the proctodeum, forms the lining of a small partof the posterior end of the alimentary canal. The lens and. Fig. 26. — Cross section of a frog embryo. AR, archenteron ; MS, meso-blastic somites ; AT, notochord ; NS, neural crest; M, medullary tube;PR, pronephros; SNt subnotochordal rod ; SO, SP, somatic and splanch-nic mesoderm. (From Morgan, after Marshall.) cornea as well as the retina of the eye, and the vesicle ofthe inner ear, also take their origin from this layer. Organs from the Entoderm. — The entoderm, or thegerm layer which is invaginated within the egg, gives rise tothe lining of the alimentary canal and of all organs which v THE DEVELOPMENT OF THE FROG 107 arise as outgrowths from it. The first of these to be formedis the liver, which at the beginning appears as an outpocket-ing of the ventral side near the anterior end. The out-pocketing becomes folded and branched, being convertedfinally into a number of clusters of tubules, all emptyinginto the common canal, the bile duct, which is producedby a lengthening of the neck of the original outgrowth. Alateral outgrowth of the
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