. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OVUM. 75 half of the diameter of the whole ovule. In the progress of growth, it enlarges some- what, but only in the earlier periods, and in less proportion than the yolk, and undergoes no farther increase during the greater part of the time that the yolk acquires the greatest addition of new matter. It is worthy of remark, however, that the germinal vesicle is originally of a large size in the eggs of birds and other large-yolked ova ; that it is also of very considerable size, even proportionally larger, in the batrach
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OVUM. 75 half of the diameter of the whole ovule. In the progress of growth, it enlarges some- what, but only in the earlier periods, and in less proportion than the yolk, and undergoes no farther increase during the greater part of the time that the yolk acquires the greatest addition of new matter. It is worthy of remark, however, that the germinal vesicle is originally of a large size in the eggs of birds and other large-yolked ova ; that it is also of very considerable size, even proportionally larger, in the batrachia ; and that in mammalia, and other animals with the smaller and gra- nular yolk, its size bears in general a propor- tion to that of the yolk. The substance of the yolk appears, in the first place, to be simply granular, or to be composed entirely of minute molecules such as those which always form the yolk in mam- malia. These are united together by a some- what glairy fluid ; larger spherules gradually appear among them ; and next the distinction between the substance of the proligerous disc and of the yolk cavity becomes apparent. Lastly, 'the deep-coloured yolk corpuscles are produced, layer after layer being deposited from the exterior, so that the outermost are the last formed. Externally a closer-set layer of nucleated cells covers the surface, in con- nection with which the vitelline membrane is formed. The vitelline membrane is not formed at an early period in the bird's egg: it cannot indeed be perceived in ovula of a tenth of an inch in diameter. We shall presently see that its relations and mode of formation are peculiar in the bird's egg. Morphology of the bird's egg as ascertained from its first origin and development. — The ovaries of the common fowl, and indeed of most large birds, are less favourable for the investigation of the first origin and earliest condition of the ovule, than those of the smaller tribes; this arises, not so much from the dense struc
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