Biology Pamphlets Volume 42 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 Excerpt: ...while the cranial nerves have retained more of the primitive characteristics. In Petromyzon, Kupffer (38) has shown that the "dorsal spinal nerve root" and the "mixed head nerve root" exist together side by side as coordinate components of a typical head nerve. In the researches of Kupffer (36), we gain important additions to our knowledge of the development of the cranial ganglia in connection with the thickenings of the ectoderm which have long been recognized, but little understood. Since Beard's (16) and Froriep's (39) simultaneous discovery of " branchial sense organs" in the embryos of sharks, and the corresponding transient structures in embryos of higher forms, there has been much controversy in regard to the question of the ectoblast elements entering secondarily into the formation of the cranial ganglia and nerves. There has been much hesitation on the part of investigators in accepting this fact, for, as Froriep (39) has said, "It would certainly bring about a fundamental change in our views, were we to be convinced that during a long period of embryonic development, the whole ectoblast possessed the capacity to act as 'Nervenkeim.'" It is now settled beyond dispute that these "placodes" in Ammocetes do furnish material to the processes growing down from the neural ridge, and subsequently forming the cranial ganglia and nerves. The peripheral portion of the "placodes" may become the "foundations of the primary sense organs." The sense organs of the lateral line, although distributed over the entire length of the trunk, are connected with ganglia formed in the head region, and are therefore innervated by cranial nerves. There seems every reason for considering the system as belo...