The Prootic Somites of Heterodontus and of Amia

THAT the prootic somites in the head of the vertebrate give rise to the muscles actuating the eyeballs is a fact so well known as to require no comment. As the result of the work of Marshall (7) it was understood that the eye-muscles, of which there are six, arise in three groups, each group from one prootic somite. The first group, consisting of the superior, internal, and inferior recti and the inferior oblique, arise from the first or premandibular somite, and are innervated by the oculomotor nerve. The second, to which the superior oblique alone belongs, is produced by the second or mandibular somite, its nerve being the pathetic. To the third, consisting of the external rectus, the third or hyoid somite gives rise, and it is innervated by the abducens. Neal (8), in a recent and careful work on S qua I n s , has, however, slightly modified this scheme. According to him the external rectus muscle has a double origin. Part of it is formed from the hyoid somite and part from the mandibular. The element contributed by the mandibular is in his opinion the posteriorly directed process of the mandibular somite named muscle E by Platt (10), its discoverer. While it is generally supposed to disappear, Dohrn (3) was of the opinion that muscle E persisted, and NeaJ