FERTILIZATION OF THE OVUM. 949 



such excretory duct, but the ova pass into the peritoneal cavity, from which 

 they escape, in the Cyclostomata, by an orifice on the under side of the hinder 

 part of that cavity, named the abdominal pore. In the sharks, a short tube, 

 or rudimentary oviduct, first receives the ovum. In a sort of chamber, con- 

 nected with this, the peculiarly-shaped, horny, protective case is secreted, like 

 a chorion, in which the ovum is discharged. 



In the Non-vertebrated animals the ovaries are neither solid and parenchy- 

 matous as in Mammalia, nor racemose as in Birds, Reptiles, Amphibia, and 

 Fishes. In the higher forms they consist of sacs, cceca, or tubuli, which may 

 be simple or ramified, and which, like a gland, have an attached or connected 

 duct, named the oviduct, which, however, is not separate from the ovary, as in 

 most Vertebrata. In the lowest forms, the ova are developed, sometimes in a 

 loose filamentous tissue, or in membranous plicae, or upon stalks or processes 

 in the interior of the body, as in the Ccelenterata, or are actually embedded in 

 its substance, as in the Protozoa. 



Mollusca and Molluscoida. In the Cephalopods, the ovaries are saccular. 

 The ova are developed upon short processes in these sacs, and, when detached, 

 leave a part behind, somewhat resembling a calyx. They are received into a 

 special chamber, in which a protective covering is superadded to them. In 

 the other Mollusca, the ovaries are found either arranged in strings, or in 

 masses in the body-cavity. In the Lamellibranchiata, the ovaries are fol- 

 licular. In the Molluscoida, the ova, developed in follicles, are discharged by 

 the oral cavity. 



Annulosa and Anmiloida. In Insects, the eggs are generally numerous ; 

 the ovaries are csecal, like the follicles of glands ; they are double and sym- 

 metrical, but have a common outlet, in a sort of cloaca ; frequently, the eggs 

 are laid by aid of an ovipositor. The females of many Insects have a reservoir, 

 known as the spermotheca, like the bee. In the Crustacea, the ovaries are 

 also double, each having its own outlet ; they form caeca, usually branched, 

 but in the lower forms, simple. The oviducts are often provided with a sper- 

 motheca. The Arachnida have elongated vesicular ovaries. In the Myria- 

 poda, they are like those of Insects. In the Annelids, the ovaries have no 

 oviducts, but the ova are set free in the perivisceral cavity. Amongst the 

 Annuloida, in the vermiform Scolecida, the ovaries are either simple, or, more 

 commonly, consist of much-ramified tubuli. The ova are numerous, and are 

 discharged from a proper outlet, or from the anal orifice. In the Teenia, the 

 ovaries are multiple, like the body ; each segment has its ramified canals ; in 

 one species, the total number of eggs, in all the segments, is said to be 64,000,- 

 000. In the Eotifera, the ovary is single and saccular ; the young are some- 

 times developed, more or less completely, within the parent animal. In the 

 Echinodermata the ovaries are ramified tubes, modified according to the 

 shape of the body of the animal, there being usually a pair in each arm or 

 segment ; but in the Holothurida, they are single, have terminal clusters of 

 caeca, and open near the mouth. 



Coelenterata. In some of these, as in the Physograde and Cirrhigrade forms, 

 the ova are developed in clusters on the base of the cirrhi. In the Pulmograde 

 forms, they are developed in sacs in the body : cavity. In the Actinozoa, they 

 adhere to plicated folds of membrane, in that cavity. There are no oviducts, 

 and the ova are discharged from the oral aperture. 



Protozoa. In these animals, the germ-cells scarcely appear like true ova ; 

 they form on, or in, the substance of the parent. 



From the preceding account, it is evident that the ovaries are homologous 

 with glands ; so that the germ-cells, or ova, may, as well as the sperm-cells, 

 be regarded as the products of a special nutrient secretive act. 



The Fertilization of the Ovum. 



The fertilization of the ovum, whether it occur within or without 

 the body of the ovigerous parent, requires the contact of the male 

 fertilizing agent, which, in many cases, has been recognized under 

 the microscope, by the actual presence of spermatozoa upon, or even 



