A treatise on the science and practice of midwifery . in different animals. Itis very abundant in birds, in whom it forms the familiar white of theegg. In some animals it has not been detected, so that its presencein the human ovule is uncertain. Where it exists it doubtless con-tributes to the nourishment of the ovule. Coincident with thesechanges is the disappearance of the germinal vesicle. At the sametime the yelk contracts and becomes more solid; retiring, in onespot, from close contact with the zonapellucida, and thus forming a species ofcavity called by Newport the respira-tory chamher,
A treatise on the science and practice of midwifery . in different animals. Itis very abundant in birds, in whom it forms the familiar white of theegg. In some animals it has not been detected, so that its presencein the human ovule is uncertain. Where it exists it doubtless con-tributes to the nourishment of the ovule. Coincident with thesechanges is the disappearance of the germinal vesicle. At the sametime the yelk contracts and becomes more solid; retiring, in onespot, from close contact with the zonapellucida, and thus forming a species ofcavity called by Newport the respira-tory chamher, which in some animals istilled with a transparent liquid. Afterthis occurs the very peculiar phenome-non known as the cleavage of the yelk,which results in the formation or themembrane from which the foetus is de-veloped. It is preceded by the forma-tion at one point of the surface of theyelk of a minute transparenl globule ofa bluish tint, sometimes of three or fourseparate globules which subsequentlyunite into one. This has received the Fig. Po matlon of the Polar Pellucida. oonl Vetlele. Yelk. •. and f. ,. The Polar Globule. OS PREGNANCY. name of the polar globule (Fig. 42), and seems to be formed from thehyaline substance of the yelk, from which it soon becomes entirelyseparated, and remains attached to the inner surface of the zonapellucida. It indicates the point at which the segmentation of theyelk begins, and where the cephalic extremity of the foetus will sub-sequently be placed. According to Eobin these changes occur in all ovules, whetherthey are impregnated or not, but if the ovule is not fecundated, nofarther alterations occnr. Supposing impregnation has taken place,a bright clear vesicle, called the vitelline nucleus, very similar inappearance to a drop of oil, appears in the centre of the yelk. Thesegmentation of the yelk (Fig. 43) commences at the point where thepolar globule is situated; it begins to divide into two halves, and at
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Keywords: ., bookcentury1800, bookdecade1870, bookidtre, booksubjectobstetrics
