948 SPECIAL PHYSIOLOGY. 



tessellated cells outside it. At first, smooth on its outer surface, it 

 speedily becomes covered over with minute, soft, scattered knobs, 

 which soon enlarge and form simple villi. These, composed of nu- 

 cleated cells only, like the outer tessellated layer of the chorion itself, 

 grow rapidly, and form swollen or club-shaped ends, which embed 

 themselves in the soft structure of the early decidua vera, from which 

 they doubtless actively absorb nourishment for the ovum. Afterwards, 

 these primitive villi are replaced by other branched or tufted villi, 

 which form the so-called shaggy or villom chorion. These latter villi, 

 after the others have disappeared, continue to enlarge, receive blood- 

 vessels proceeding from the embryo now forming within the ovum, and 

 so produce vascular processes or tufts, which project, or depend, into 

 the venous sinuses or lacunae of the maternal portion of the placenta. 

 They constitute the embryonal or foetal portion of the placenta, and, 

 coming into close relation with the maternal blood, are the organs by 

 which the nutrition and respiration of the embryo are henceforth 

 carried on. 



The chorion itself soon becomes lined with another membrane, named 

 the amnion, which, as will afterwards be described, is derived from the 

 embryo, and contains a fluid, the liquor amnii, which serves to protect 

 the foetus in utero, until the moment of its birth. The Mammalia are 

 called viviparous animals. 



The Ovaries and Ova of other Animals. 



The description of the ovum of Mammalia and Birds applies, in most respects, 

 to the ova of the other Vertebrata ; but there are certain peculiarities in some 

 of these, and in the ovaries in which they are formed. The same parts in the 

 Non-vertebrate animals, also require notice. 



Beptilia. In the Reptiles, which like Birds are oviparous, the ovary is also, 

 as in them, racemose, and, as a rule, single. The yolk is large, and covered 

 with an abundant white, inclosed in a shell ; but this is soft, instead of being 

 firmly calcified. When the yolk is formed, the ova escape, by dehiscence, into 

 the abdominal cavity, and are afterwards received into the oviduct, which is 

 placed at a considerable distance higher up. By these, as in Birds, they are 

 discharged into the cloaca, and thence are generally deposited externally, or 

 oviparously. In the viper, the slow-worm, and green lizard, however, the de- 

 velopment'of the embryo takes place partially within the body of the parent ; 

 hence such reptiles are said to be ovoviviparous. 



Amphibia. In the Amphibia, the ovaries are double, and the ova are no 

 longer, as in the Mammalia, Birds, and Reptiles, brought to maturity in suc- 

 cession, but simultaneously; being received into the oviducts, they are conveyed 

 to the cloaca, and are then deposited in the water, either singly, in chains, or 

 in masses. They are surrounded by a soft, mucous, areolar tissue, which 

 swells up in the water, keeps the ova apart, allows light and aerated water to 

 get between them, and supplies temporary food to the young tadpoles. 



Fishes. In Fishes, the ovaries are also double and symmetrical, and are 

 chiefly remarkable for the enormous number of ova developed in them. The 

 number of ova in a codfish has been found to be upwards of 3,500,000, in a 

 flounder, 1,300,000, and in a mackerel, more than 500,000. They are usually 

 matured and deposited simultaneously, but in the case of migratory sea-fishes, 

 like the herring, probably at successive periods. In certain Fishes which are 

 ovoviviparous, the ova are few in number, and are deposited at short intervals. 

 Most commonly the ovaries have an excretory duct, continuous with them, like 

 the duct of a gland, by means of which the ova are discharged into the water. 

 The Cartilaginous, and a small number of Osseous Fishes, however, have no 



