. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OVUM. [87] more firmly united to wards the external surface. This circumstance lias given rise to the belief among some observers in the existence of an additional delicate membrane enclosing the yolk-mass ; but the most attentive observa- tion by Bischoff, Wharton Jones, myself, and others has failed to detect such a membrane ; and there is reason to think that the con- fident belief in its existence has had its origin in part at least in a desire to establish a more complete analogy between the ovum of birds and Mammal
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OVUM. [87] more firmly united to wards the external surface. This circumstance lias given rise to the belief among some observers in the existence of an additional delicate membrane enclosing the yolk-mass ; but the most attentive observa- tion by Bischoff, Wharton Jones, myself, and others has failed to detect such a membrane ; and there is reason to think that the con- fident belief in its existence has had its origin in part at least in a desire to establish a more complete analogy between the ovum of birds and Mammalia, and to find accordingly a vitelline membrane as well as a chorion present in the ova of the latter. The germinal vesicle is usually about a sixth of the diameter of the whole ovum ; but it is sometimes larger, or between a filth and a fourth. It possesses a delicate membranous wall of a spherical or spheroidal form and ho- mogeneous structure : it is barely possible to observe the double line of the thickness of this wall with the quarter of an inch lens in the microscope. In most animals the ger- minal vesicle is readily distinguishable from the rest of the ovum by its superior clear- ness, excepting in those instances in which it is hidden by the great opacity of the yolk- substance; but then it may generally be made manifest by flattening the ovum by compres- sion between plates of glass. The fluid which fills its cavity, which is generally very clear, contains some minute granules in suspension; and besides these there is apparent within it the macula germinativa or germinal nucleus. This last, which is in general well defined in the mammiferous ovum, varies slightly in different animals: in some presenting the appearance of a round globule, with a deli- cate circumscribing line almost amounting to a vesicular covering ; but more frequently it consists only of a small spherical or dis- coid mass of fine granules. In a germinal vesicle of ^1^" in diameter, such as
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