. The development of the chick; an introduction to embryology . primordia of the cerebral gan-glia, and siniilai- successively arising enlargements of the partsof the crest opposite the mesoblastic somites form the rudimentsof the spinal ganglia. The intervening portions of the crest formthe so-called interganglionic commissures, which subsequently FROM TWELVE TO THIRTY-SIX SOMITES 157 appear to form mesenchyme. The formation of mesenchymefrom certain parts of the neural crest is most marked in theregion of the brain. The primordia of the ganglia contain the cells (neuroblasts)which form the d
. The development of the chick; an introduction to embryology . primordia of the cerebral gan-glia, and siniilai- successively arising enlargements of the partsof the crest opposite the mesoblastic somites form the rudimentsof the spinal ganglia. The intervening portions of the crest formthe so-called interganglionic commissures, which subsequently FROM TWELVE TO THIRTY-SIX SOMITES 157 appear to form mesenchyme. The formation of mesenchymefrom certain parts of the neural crest is most marked in theregion of the brain. The primordia of the ganglia contain the cells (neuroblasts)which form the dorsal root fibers of the spinal nerves and partsof certain cranial nerves. They also appear to contain the cellsfrom which the sheaths of the nerve fibers are formed; thusthree kinds of cells at least are found in the neural crest, viz.,mesenchyme forming cells, neuroblasts, and sheath cells. The Cranial Neural Crest and its Derivatives. The neuralcrest in the head may be divided into pre- and post-otic divisions,and these arise at different times. fCr ^iiUe. ^^^^^te^r^^ Fig. 90. — Transverse section of the fore-brain, and opticvesicles at the stage of 7 , Mesenchyme, n. Cr., Neural crest. Ph., Phar-ynx. Snt. cer., Anterior cerebral suture. X., Mass of cellsin which the anterior end of the intestine, the neural tubeand the notochord fuse. (1) The pre-otic division, which extends from the extremeanterior end of the neural tube to about the center of the audi-tory pit, is well developed at a stage of 7-8 somites, but it is notfound at the 5 s stage. The origin is everywhere the same, viz.,from the dorsalmost cells of the medullary plate and the ecto-derm innnediately adjacent; it arises at the time of contact ofthe medullary folds and is thus thickest in the region of thesuture. Fig. 90 is a section throtigh the developing optic vesicles,a2id shows the neural crest continuous with the tube and ectoderm 158 THE DEVELOPMENT OF THE CHICK in the neural suture; it is separated
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