The frog: an introduction to anatomy, histology, and embryology . oncerned with the appre-ciation of the presence and nature of external objects, and aretherefore necessarily formed on the surface of the body. Theymay be regarded as specially modified portions of the epidermis. The Nose. The olfactory organs appear at a very early stageas paired thickenings of the nervous layer of the epiblast at 122 DEVELOPMENT OF THE PROG the anterior end of the head, in the angles between the fore-brain and the optic vesicles. A pitting-in of the surface,involving both layers of the epiblast, soon appears i
The frog: an introduction to anatomy, histology, and embryology . oncerned with the appre-ciation of the presence and nature of external objects, and aretherefore necessarily formed on the surface of the body. Theymay be regarded as specially modified portions of the epidermis. The Nose. The olfactory organs appear at a very early stageas paired thickenings of the nervous layer of the epiblast at 122 DEVELOPMENT OF THE PROG the anterior end of the head, in the angles between the fore-brain and the optic vesicles. A pitting-in of the surface,involving both layers of the epiblast, soon appears in each ofthese thickenings, and the pits so formed become the nasalsacs ; the mouths of the pits forming the nostrils or anteriornares, and the epiblast lining the pits giving rise to the olfac-tory epithelium. Erom the inner or deeper end of each olfactory pit a diverti-culum, at first solid, but soon becoming hollow, gi-ows down-wards to the roof of the pharynx, into which it opens,^ theposterior nares, very shortly after the formation of the Fig. 30.—Half sections in the transverse plane of a tadpole 10 (left half) and of a tadpole 12 mm. long (right half). X 35. BF, fore-brain ; OD, outer wall of optic cup (pigment layer of adultretina); OC, inner wall of optic cup (remainder of adult retina);OL, lens, attached to epiblast in younger tadpole, but forming ahollow vesicle at the later stage ; TP, pharynx ; Q, sucker. [G. H. F.] The Eye. The eye differs from the other sense organs, inas-much as the lens alone is formed directly from the surface epi-blast, while the sensitive part of the eye, or retina, arises as anoutgrowth from, the brain. The optic vesicles have already beendescribed as arising at a very early period as lateral outgrowthsfrom the fore-brain; these soon become constricted at theirnecks so as to be connected with the brain by narrow stalks,which ultimately become the optic nerves. The outer surface of each optic vesicle, which is at
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbiology, bookyear1896
