. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. GENERAL CHARACTERS OF THE VERTEBRATA. 347 dbrsales;" and they become more and more raised up, till they ultimately meet in the middle line, and unite to form a tube, within which the cerebro-spinal nervous centres are developed. It follows from its mode of formation that the inner wall of the tube formed by the primitive groove, which remains as the septum between the cerebro-spinal canal and the body-cavity, is nothing more than a portion of the primitive wall of the body o
. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. GENERAL CHARACTERS OF THE VERTEBRATA. 347 dbrsales;" and they become more and more raised up, till they ultimately meet in the middle line, and unite to form a tube, within which the cerebro-spinal nervous centres are developed. It follows from its mode of formation that the inner wall of the tube formed by the primitive groove, which remains as the septum between the cerebro-spinal canal and the body-cavity, is nothing more than a portion of the primitive wall of the body of the embryo. And there appears to be little doubt, as be- lieved by Remak and Huxley, that the cerebro-spinal nervous centres are "the result oT a modification, of that serous layer of the germ, which is continuous elsewhere with the epidermis " (Huxley).. Fig. 124.âEmbryology of Vertebrata. A, Portion of the genninal area of the ovum of a Bitch, showing the primitive groove (after Bischoff). ^ B, Profile view of the same. C. Diagram representing the amnion and allantois: e Embtyo; a Am- nion : » tJmbilical vesicle ; ^Allantois; f Pedicle of the allantois, afterwards the urinary bladder. â D, Head-of an embryo, showing the visceral arches ~(z' 'if^. Another remarkable peculiarity as regards the nervous sys- tem is found in the fact that in no Vertebrate animal does the ahmentary canal pierce the main masses of the nervous system, but turns away to open on the opposite side of the body. In most Iiivertebrates, on the other hand, in which there is a well-developed nervous system, this is perforated by the gullet, so that an oesophageal nerve;Collar is formed, and some of, the nervous centres become pree-cesophageal, whilst others are post-oesophageal. Furthermore, the flopr of the " primitive groove" in the embryo of all Vertebrates has developed in it at an early period the structure known as the "notochord" or"qhorda dorsalis" (fig.
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