50 



OVUM. 



the eel among the osseous fishes, the oviduct 

 is entirely wanting, and the numerous ova 



which are dischargee 



by external dehiscence 



from the ovary into the cavity of the 

 abdomen, escape from that cavity by an 

 orifice (porus abdominalis) situated on each 

 side close to the anus. 4th. In other osseous 

 fishes, the ovary and oviduct are united, 

 or the ovary forms a saccular organ, in 

 the interior of the wall of which the ovi- 

 capsules are situated, occupying a variable 

 extent of it in different genera ; and the wall 

 of the oviduct, usually very short, is continued 

 from that of the ovary to the outlet from the 

 animal's body. The ova, therefore, which 

 drop by internal dehiscence into the cavity of 

 the ovary, pass directly out by the short ovi- 

 duct in the laying of the spawn. Most osseous 

 fishes are oviparous ; but in a few, as the 

 viviparous blenny, the anableps, paecilia, and 

 some siluroids, the ova, on escaping from their 

 capsules into the cavity of the ovary, remain 

 there during the development of the embryo. 

 In the invertebrate animals there are very 

 many varieties in the form and relations of 

 the productive and conducting parts of the 

 organs. Three principal 



female generative 



Fig. 40. 



Oviduct and ovary in a continuous tube in Insects 

 and Entozod. 



A. (From R. Wagner}. Upper part of the ovi- 

 duct or ovary of the Acheta campestris. 



B. (From H. Nelson.} Upper part of the oviduct 

 or ovary of the Ascaris mystax. In both of these 

 figures the germ -cells and germinal vesicles, with 

 their nuclei, are seen surrounded by the granular 

 matter which afterwards collects round them as 

 vitelline or yolk substance. 



varieties may be distinguished among them 

 1 st. A form similar to that just now described 

 as generally prevalent among osseous fishes, 

 in which the ovary and oviduct are con- 

 tinuous, but in which the ova, being formed 

 in ovarian capsules, are dropped by dehiscence 

 into the upper part of the oviducts. Such is 

 still the structure in cephalopoda and some 

 other mollusca. 2nd. A form in which the 

 oviduct may be said to be, as in the last, con- 

 tinuous with the ovary, but in which there 

 is no true dehiscence of the ovules from 

 ovarian capsules, as they are formed at once 

 in the internal cavities of the ovary, which 

 directly open into, or are mere prolongations 

 of, the oviducal tubes. In this form the 

 oviducts may be considered to stand in the 

 relation of excretory ducts to the ovarian 

 glands. In many of this class the ovaries 

 present very various forms ; in some the 

 continuity of the ovarian and oviducal tubes 

 is very obvious and simple, as in the ne- 

 matoid entozoa, insects, &c. ; while in others, 

 the ovary is more complex and race- 

 mose, and the oviducal tubes comparatively 

 simple. 3rd. That form in which the ovaries 

 are variously disseminated over the body of 

 the animals, and in which there are no true 

 oviducts, but the ova escape on various parts 

 of the internal or external surface of the 

 body.* 



b. Structure of the ovaries themselves, as 

 related to the production of the ovitla. In 

 mammalia these organs consist of a pair of 

 solid oval flattened bodies, attached by inter- 

 vening fibrous tissue to the posterior surface 

 of the broad ligaments of the uterus, and are 

 covered completely, excepting at this attached 

 part, by peritoneum. Below this serous co- 

 vering there is also a layer of firm fibrous 

 tissue, or tunica albuginca. The internal sub- 

 stance, or parenchyma, or stroma, as it has 

 been called, consists of a firm basis of fibro- 

 cellular texture, of considerable vascularity. 

 The fibres, as well as the blood-vessels of this 

 substance, radiate principally from the at- 

 tached border of the organ towards the oppo- 

 site, or free side, and the rest of the surface. 

 The ovicapsules, or so-called vesicles or fol- 

 licles of De Graaf, in the human ovary, are 

 situated in this stroma; and at or after the 

 period of puberty are found of some size ; 

 a variable number, from twelve to thirty, or 

 more, being of from J^ to | of an inch, and 

 a few even a little larger. These mem- 

 branous vesicles, filled with fluid, are situated 

 chiefly towards the surface of the free side of 

 the ovary. A larger number of undeveloped 

 capsules, of minute size, also exist in the 



* See Von Baer's Entwickelungsgesch. der Thiere ; 

 Owen's Lectures on Invertebrate Animals, 1843, anil 

 on Fishes, 1846 ; Eathke (on Development of 

 Fishes, &c.), in Geschichte der Thierwelt, Th. 3. ; 

 J. Miiller (on Sharks), in Mem. of Berlin Acad. 

 1842 ; John Davy (on the Torpedo), in Philos. 

 Trans, for 1834; and the works of Von Siebold and 

 Stannius, R. Wagner, Cams, and others on Compar. 

 Anat. See also in this Cyclopaedia, the articles 

 Monotremata, Pisces, Reptilia, and Organs of Ge- 

 neration. 



