SUPPLEMENT. 



OVUM. In Animal Anatomy and Physi- 

 ology, the Egg, the product of parental sexual 

 generation, from which the young of animals 

 are produced. 



The Functions of Reproduction, as observed 

 in the higher orders of animals and in the 

 human species, are generally divided into two 

 classes of processes ; the one of which com- 

 prehends those operations by which the 

 parents contribute to the production of the 

 germs from which the young are formed ; the 

 other, those processes or changes which occur 

 more immediately in the product of genera- 

 tion itself, and which relate to the formation 

 or development of the new being from a germ 

 or ovum. In the Article GENERATION of this 

 Cyclopaedia, the functions belonging to the 

 first of these divisions have been described ; 

 and it is proposed in the present Article to 

 treat of the second class of reproductive phe- 

 nomena, or those which relate more imme- 

 diately to the origin, formation, and growth of 

 the new bettig, and which are usually described 

 under the titles of Ovology, Embryology, and 

 Foetal Development. 



In this, as in the former article, the history 

 of the functions as they occur in the human 

 species will receive the greatest share of our 

 attention ; but in describing the process of 

 development of the young, still more than in 

 the history of the functions of the parents 

 that are preliminary to the production of a 

 perfect germ, it is necessary to extend our 

 observations to the various members of the 

 animal kingdom, and even in some degree also 

 to plants, from which, as much as from direct 

 observations or experiments in man, has been 

 derived our knowledge of the individual facts 

 and of the general laws relating to the process 

 of embryonic development. 



The arrangement followed in that part of 

 the article which treats of Development will 

 be adapted more immediately to the consi- 

 deration of human reproduction ; and the 

 statements in regard to animals, or to organ- 

 ised beings in general, will be made chiefly 

 subordinate to, or illustrative of, the functions 

 in the human species ; but the facts in human 

 and comparative embryology are so intimately 

 connected, that it will be expedient to incor- 

 porate with the article such a description of 

 the formative process in different animals as 

 may present a sketch of the general nature of 



Supp. 



this interesting process in the whole animal 

 kingdom.* 



In pursuing this plan, the topics to be dis- 

 cussed may be arranged under the following 

 heads : viz. 



Jst. Nature of the Ovum in general, with 

 reference to the different forms of the repro- 

 ductive function in various animals. 



2nd. The structure, properties, mode of 

 origin, and formation of the Ovum. 



3rd. The changes which the ovum under- 

 goes in the process of Fecundation, and (in so 

 far as the ovum itself is concerned) the cir- 

 cumstances which influence that process. 



4th. The external circumstances which in- 

 fluence the development of the ovum and 

 embryo, especially Incubation and Utero- 

 gestation. 



5th. The Phenomena of Foetal Development 

 in general, and the history of the origin and 

 development of each system, organ, and tex- 

 ture of the body in particular. 



6th. The Functions of the Embryo or 

 Foetus as compared with those of the adult. 



The wide-spread importance of embryolo- 

 gical anatomy and physiology is now so 

 generally acknowledged by all who have 

 made them a subject of study, that to them 

 no apology is required for the length of this 

 treatise. To those who have not made them 

 an object of their special attention, it will be 

 enough at this place to advert to the exten- 

 sive range of topics which must be embraced 

 in an attempt to trace the history of the 

 first origin and subsequent evolution of all 

 the parts of so complex and various a struc- 

 ture as the body of animals ; and to remind 

 them that this department of science pro- 

 fesses to describe not merely the successive 

 changes of external form and relation by 

 which the several organs, springing from im- 

 perceptible beginnings, arrive at their perfect 

 condition, but also the more minute pheno- 

 mena of histological development, or changes 

 of the several textures, which accompany the 

 more obvious formative processes ; that, as in 

 many instances the complete knowledge of 



* It was originally intended to have treated in 

 the same article of the embryology of plants ; but 

 the extent and importance of that subject in con- 

 nection with general physiology makes it necessary 

 to postpone its consideration to a separate article, 

 under the head of VEGETABLE OVUM. 



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