. A text-book of animal physiology [microform] : with introductory chapters on general biology and a full treatment of reproduction, for students of human and comparative (veterinary) medicine and of general biology. Physiology, Comparative; Veterinary physiology; Physiologie comparée; Physiologie vétérinaire. REPKODUCTION. 67 this region becoming more columnar (histological differentia- tion). This depression {invagination) deepens until a cavity is. Fio. and gastrula of amphioxus (Claus, after Hatschek). A, bl^tula with flat- tened lower poJe of brger cells; B, commenclne invaK
. A text-book of animal physiology [microform] : with introductory chapters on general biology and a full treatment of reproduction, for students of human and comparative (veterinary) medicine and of general biology. Physiology, Comparative; Veterinary physiology; Physiologie comparée; Physiologie vétérinaire. REPKODUCTION. 67 this region becoming more columnar (histological differentia- tion). This depression {invagination) deepens until a cavity is. Fio. and gastrula of amphioxus (Claus, after Hatschek). A, bl^tula with flat- tened lower poJe of brger cells; B, commenclne invaKinatipn; C, gaBtnUatlon completedi; the blastopore is stlU widely open, and one of flie two hinder-pole mesoderm cells Is seen at its ventral Up. The cilia of the epiblast cells are not represented. formed (as when a hollow rubber ball is thrust in at one part till it meets the opposite wall), in consequence of which a two- layered embryo results, in which we recognize the primitive ifaouth {blastopore) and digestive cavity {archenteron), the outer layer {ectoderm) being usually separated from the inner {endoderm) by the almost obliterated segmentation cavity. Such a form may be provided with cilia, be very actively loco- motive, and bear, consequently, the greatest resemblance to the permaiient forms of some aquatic animals. The changes by which the segmented oosperm becomes a gastrula are not always so direct and simple as in the above- described case, but the behavior of the cells of the blastosphere may be hampered by a burden of relatively foreign matter, in the form of food-yelk, in certain instances; so much so is this the case that distinct modes of gastrula formation may be rec- ognized as dependent on the quantity and arrangement of food- yelk. These we shall pass by as being somewhat too compli- cated for our purpose, and we return to the egg of the bird. The Hen'i Egg.—By far the larger part of the hen's egg is made up of yelk; but just beneath the vitelline me
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Keywords: ., bookauthormillswes, bookcentury1800, bookdecade1880, bookyear1889
