. A text-book of comparative physiology [microform] : for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative; Veterinary physiology; Physiologie comparée; Physiologie vétérinaire. REPRODUCTION. 71 rm) of the >roduction, he appear- ff 08 above vhich they jroung but (atroma), Uliker). I hang ^6 opment, so f grapes ia an early stage. The ovum at first, in this case as in all others, a single cell, becomes complex by addition of other cells (dia- eua proligerua, etc.), which go to make up tlie yelk. All the other parts of the hen's egg are additions made to
. A text-book of comparative physiology [microform] : for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative; Veterinary physiology; Physiologie comparée; Physiologie vétérinaire. REPRODUCTION. 71 rm) of the >roduction, he appear- ff 08 above vhich they jroung but (atroma), Uliker). I hang ^6 opment, so f grapes ia an early stage. The ovum at first, in this case as in all others, a single cell, becomes complex by addition of other cells (dia- eua proligerua, etc.), which go to make up tlie yelk. All the other parts of the hen's egg are additions made to it, as ex- plained before, in its passage down the oviduct. The original ovum remains as the blastoderm, the segmentation of which may now be described briefly, its character being obvious from an examination of Fig. 68, which represents a surface view of the segmenting fertilized ovum (oosperm). A segmentation cavity apv^ears early, and is bounded above by a single layer of epiblast cells and below by a single layer of primitive hypoblast cells, which latter is soon composed of sev- eral layers, while the. segmentation cavity disappears. The blastoderm of an unincubated but fertilizei! egg consists of a layer of epiblastic cells, and beneath this a mass of rounded cells, arranged irregularly and lying loosely in the yelk, consti- tuting the primitive hypoblast. After incubation for a couple of hours, these cells become differentiated into a lower layer of flattened cells {hypoblast), with mesoblastic cells scattered be-. Fio. 99.—Portion of section throngh an nnlneubated fowl's oOapenn tafter Klein), a. epIblMt composed of • single layer of colnninar cells; b, inegnlarly disposed lower lajrer cells of the primitive hypoblast; e, larger formative cells resting on white yelk; /, aichenteron. The segmentation cavity lies between a and b, and is nearly obliterated. tween the epiblast and hsrpoblast. It is noteworthy that, in the bird, segmentation will proceed up to a cert
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