. The biology of the frog . contact with another structure, the pituitarybody, which is developed from the ectoderm of the dorsalwall of the stomodeum. The sides of the thalamencephalongive rise to a pair of lateral diverticula, the optic vesicles,which grow out until they come in close contact with thesurface ectoderm. The distal end of the vesicles widensout to form the retina of the eyes, the stalk giving rise tothe optic nerve. The anterior wall of the forebrain produces a pair ofpouches, the cerebral hemispheres, which finally becomethe largest part of the brain. Their cavities, the later
. The biology of the frog . contact with another structure, the pituitarybody, which is developed from the ectoderm of the dorsalwall of the stomodeum. The sides of the thalamencephalongive rise to a pair of lateral diverticula, the optic vesicles,which grow out until they come in close contact with thesurface ectoderm. The distal end of the vesicles widensout to form the retina of the eyes, the stalk giving rise tothe optic nerve. The anterior wall of the forebrain produces a pair ofpouches, the cerebral hemispheres, which finally becomethe largest part of the brain. Their cavities, the lateral ven-tricles, communicate with the third ventricle by an opening,the foramen of Monro. The nerves arise as paired outgrowths both from the brainand cord, pushing their way between the cells of the otherorgans, dividing and ramifying, as they push outwardtoward the various parts they supply. The spinal nervesbegin as two independent outgrowths, representing thedorsal and ventral roots; these soon unite into a 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
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