The neural plate extends caudally beyond the site of the neurenteric canal. The site of the initial closure of the neural groove is rhombencephalic, upper cervical, or both. In transverse sections of the eel the large cells of the praeopticus nidulus which lie between the optic tracts give off large, deeply stained, unmeduUated fibres which pursue a lateral course with those of the ependyma toward the lateral walls, i. As compared with the previous stage, the prosencephalon has increased in length, the mesencephalon has remained the same, the rhombencephalon has decreased, and the spinal part of the neural plate has increased fivefold in length. These cells also resemble those of the mesencephalic root of the fifth and the cells of origin of Mauthners fibres. An indication of invagination of the otic disc appears towards the end of the stage. The optic sulcus is found in D1 but does not extent into D2, as has been claimed in the literature. It is proposed that the remainder of the forebrain comprises two subdivisions: D1, which becomes largely the optic primordium during stage 10, and D2, which is the future thalamic region. Graphic reconstructions have permitted the detection of the telencephalic portion of the forebrain, for the first time at such an early stage. The primitive streak is still an important feature. During this stage, the following features appear: terminal notch, optic sulcus, initial formation of neural tube, oropharyngeal membrane, pulmonary primordium, cardiac loop, aortic arches 1-3, intersegmental arteries, and laryngotracheal groove. Constantly present are the prechordal and notochordal plates (the notochord sensu stricto is not yet apparent), the neurenteric canal or at least its site, the thyroid primordium, probably the mesencephalic and rhombencephalic neural crest and the adenohypophysial primordium. The characteristic feature of this stage is 4-12 pairs of somites. Thirteen embryos of stage 10 (22 days) were studied in detail and graphic reconstructions of most of them were prepared.
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