A treatise on the science and practice of midwifery . da. At a laterperiod another similar mass is are the polar globules (Fig. 46), theorigin of which is thought to be in con-nection with the disappearance of the ger-minal vesicle and the germinal spot. Thesechanges occur in all ovules, whether theyare impregnated or not, but if the ovuleis not fecundated, no further alterationsoccur. Supposing impregnation has takenplace, a bright, clear vesicle, called thevitelline nucleus, very similar in appearanceto a drop of oil, appears in the centre ofthe yelk. After this occurs the ver
A treatise on the science and practice of midwifery . da. At a laterperiod another similar mass is are the polar globules (Fig. 46), theorigin of which is thought to be in con-nection with the disappearance of the ger-minal vesicle and the germinal spot. Thesechanges occur in all ovules, whether theyare impregnated or not, but if the ovuleis not fecundated, no further alterationsoccur. Supposing impregnation has takenplace, a bright, clear vesicle, called thevitelline nucleus, very similar in appearanceto a drop of oil, appears in the centre ofthe yelk. After this occurs the very peculiar phenomenon known asthe cleavage of the yelk, which results in the formation of the layer ofcells from which the foetus is developed. The segmentation of the yelk(Fig. 47) occupies in mammals the whole of its substance. In birds thecleavage is confined to a small area of the yelk called the cicatricula orblastoderm. Hence the term holoblastic has been applied to the ova ofmammals, mesoblastic to those of birds. It divides at first into two. Formation of the Polar pellucida, containing Yelk. 3 and 4. Germinal vesicle. polar globule.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
