The neural plate in dog embryos forms cranial to the primitive streak by 15 days and elongates as the primitive streak regresses. The neural folds contact each other at the third rhombomere at the seven somite stage. Neural crest cells are present in the early somite embryos from the optic primordia throughout the brain. Differentiation of these cells results in the anlagen of cranial nerve ganglia. Regional differentiations of the neural plate establish the prosencephalon as a broad cranial segment by four somites, with a closed neural tube, optic vesicles and infundibulum by 15 somites. The mesencephalon is an almost spherical vesicle and the rhombencephalon has become subdivided by 15 somites into seven neuromeres. The cranial flexure begins in the mesencephalon at ten somites, and the cervical flexure at the fourth somite level by 20 somites. The pontine flexure first appears at 32 somites. Secondary divisions of prosencephalon and rhombencephalon from a typical five vesicle brain anlage. The telencephalon is present at 20 somites as a bulge from the roof of the diencephalon. Telencephalic vesicles appear by 30 somites. The mesencephalon elongates slowly but is not divided into neuromeres. The rhombencephalon by 25 days has differentiated into the primordia of the cerebellum and pons cranially (the metencephalon) and the medulla oblongata caudally (the myelencephalon).
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