. A text-book of comparative physiology [microform] : for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative; Veterinary physiology; Physiologie comparée; Physiologie vétérinaire. 100 COMPARATIVE PHYSIOLOGY. transverse division of the columns of mesoblast that formed the vertebral plates. Before the permanent vertebrsB are formed, a reunion of the original protovertebrsB takes place as one cartilaginous pillai', followed by a new segmen- /•'*• tation midway between the original divisions. It is thus seen that a laige number of structures eitiier appear or
. A text-book of comparative physiology [microform] : for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative; Veterinary physiology; Physiologie comparée; Physiologie vétérinaire. 100 COMPARATIVE PHYSIOLOGY. transverse division of the columns of mesoblast that formed the vertebral plates. Before the permanent vertebrsB are formed, a reunion of the original protovertebrsB takes place as one cartilaginous pillai', followed by a new segmen- /•'*• tation midway between the original divisions. It is thus seen that a laige number of structures eitiier appear or are clearly outlined during the first day of incubation : the primitive streak, primitive groove, medullary plates and groove, the neural ca- nal, the head-fold, the cleavage of the mesoblast, the protovertebras. with traces of the anmion and areaopaca. During the second day nearly all the remaining important structures of the chick are marked out, while those that arose during the first day have progressed. Thus, the medullary folds close ; there is an increase in the number of protover- tebrse ; the formation of a tubular heart and the great blood-vessels; the appear- ance of the Wolffian duct; the progress of the head re- gion ; the appearance of the three cerebral vesicles at the anterior extremity of the neiwal canal; the subdivision of the first cerebral vesicle into the optic vesicles and the beginnings of the cerebrum ; the auditory pit arising in the third cerebral vesicle (hind-brain); cranial flex- ure commences; both head and tail folds become more dis- tinct ; the heart is not only formed, but its curvature becomes more marked and rudiments of auricles arise; while outside. Fia. 103.—Embryo of chick, between tiiirty and thMy-tix honn, viewed from above as an opaque object (Foster and Balfour). /. b, forebrain; m. ft, midbrain; A. b, hind- Drain; cp. V, optic vesicle; , anditory pit; , vitelline vein; p. v, mesoblastic somite; line of functioo o
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