. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 396 MONOTREMATA. thorhynchus, is of a spherical form, and very nearly fills the ovisac. The diameter of the germinal vesicle is to that of the ovum as 1 to 38. The vitelline fluid is rich in the number of its nucleated cells or granules, and the intermixed, clear, colourless oil-globules. The vitelline membrane is moderately thick, smooth, and highly refracting. The ovum is se- parated from the ovarian vesicle, or lining membrane of the ovisac, by a very small quantity of fluid and a stratum of granules or cells. The pro
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 396 MONOTREMATA. thorhynchus, is of a spherical form, and very nearly fills the ovisac. The diameter of the germinal vesicle is to that of the ovum as 1 to 38. The vitelline fluid is rich in the number of its nucleated cells or granules, and the intermixed, clear, colourless oil-globules. The vitelline membrane is moderately thick, smooth, and highly refracting. The ovum is se- parated from the ovarian vesicle, or lining membrane of the ovisac, by a very small quantity of fluid and a stratum of granules or cells. The proper tunic of the ovisac consists of a dense and very vascular layer of the ' stroma' or proper tissue of the ovary, which is rather thicker and more distinctly laminated than in most Mammalia, and in this re- spect widely differs from the lax stroma of the ovary of the bird. The most important difference to be noted in the present comparison with the bird is the small size of the ovarian ovum, depending on the relatively scanty amount of vitelline matter, superadded in the ovary to the essential part of the ovum, the vesicula germmatha. It may be objected that the impregnated ovarian ova in birds rapidly augment in bulk as the time of dehiscence approaches, and that, although the ovaria of the Ornithorhynchus may have been investigated within a few days of the reception of the impregnated ovum into the oviduct, the changes occurring in such a period might much more nearly approximate the ovarian ovum of the' Ornithorhynchus to the size and other conditions of that of the bird than in the instances above described. The following observations on the impreg- nated ovum in the uterus itself prove, how- ever, that no such approximation to the bird in regard to the proportion of yolk added to the ovarian ovum, or as respects the size of the ovum prior to dehiscence, is made by the Ornithorhynchus. For the acquisition of this important evi- dence in the question of the generation of
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