. Embryology. Embryology; Embryology; genealogy. NEURULA. Fig. 50. Two hearts in a single embryo. The left figure shows the two blocks of heart mesoderm which normally fuse to form a single heart. If a barrier is placed between the two halves of the heart mesoderm, they fail to fuse and each half develops into a heart, as in the right figure. 44). There is a limb field in the mesoderm; a lens field in the ectoderm; an ear field, also in the ectoderm; a field for the olfactory placode, again in the ectoderm; a heart field in the mesoderm; a hind-limb field in the mesoderm. All of these fields t
. Embryology. Embryology; Embryology; genealogy. NEURULA. Fig. 50. Two hearts in a single embryo. The left figure shows the two blocks of heart mesoderm which normally fuse to form a single heart. If a barrier is placed between the two halves of the heart mesoderm, they fail to fuse and each half develops into a heart, as in the right figure. 44). There is a limb field in the mesoderm; a lens field in the ectoderm; an ear field, also in the ectoderm; a field for the olfactory placode, again in the ectoderm; a heart field in the mesoderm; a hind-limb field in the mesoderm. All of these fields that have been examined show a labile type of organiza- tion which can be modified by experimental treatment. The nature of a field is very difficult to understand, but it does have certain quantitative characteristics which have been demonstrated particularly well in the case of the ear field. The ear arises as an ectodermal vesicle called the otocyst. The potencies of the ectoderm around the ear region may be explored by transplanting small pieces of the region to the ear region of another embryo. Usually these pieces of ectoderm will form a typical pear- shaped otocyst. Careful exploration of the whole region around the normal site of the ear indicates that a greater number of normal ear vesicles is obtained if transplants are made from or near the normal ear region. On the other hand, if transplants are made from areas farther and farther away from the normal ear region, fewer vesicles are obtained and these vesicles are more and more abnormal in structure. The field thus appears to vary in potency in a quantitative way, so that in the center of the field perfect ear vesicles are obtained in about 100 percent of the cases, whereas farther out from this. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bart
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Keywords: ., boo, bookcentury1900, booksubjectembryology, booksubjectgenealogy
