OVUM. 



the structure and function of an organ is only 

 to be obtained by the observation of its foetal 

 conditions, the study of development is acces- 

 sory or supplementary to many departments 

 of anatomy and physiology ; that, in recent 

 times, no branch of inquiry relating to organic 

 nature has made more rapid progress, has 

 presented a greater amount of new dis- 

 coveries, or has influenced in a greater 

 degree the views of scientific men on allied 

 subjects, than the science of embryology ; 

 that it is coextensive with, and illustrative of, 

 the whole range of comparative anatomy ; 

 that no system, therefore, of zoological classi- 

 fication can be regarded as philosophical or 

 complete which neglects the facts and princi- 

 ples of foetal development: finally, that some 

 departments of pathological anatomy receive 

 considerable illustration from our science, 

 and that more especially the scientific study 

 and comprehension of teratology or congenital 

 malformations is founded entirely on an accu- 

 rate knowledge of the phenomena and laws of 

 development. Our subject, therefore, is not 

 only interesting by itself, but deeply important 

 as an essential branch of philosophical ana- 

 tomy and physiology.* 



Before proceeding with the particular his- 

 tory of the ovum, and its development in man 

 and the higher animals, which will form the 

 greater part of the following article, some 

 topics of a general and preliminary nature 

 present themselves for our consideration. 



The investigation of the process of repro- 

 duction in the lower animals has made so 

 much progress during the last few years, that 

 it becomes necessary to place before the 

 reader a sketch of the aspect in which more 

 modern researches enable the physiologist to 

 view the relation of the ovum to the sexual 

 generative function, and to the other means 

 by which individuals are multiplied, or species 

 are reproduced in the whole animal kingdom. 

 In the Article GENERATION, the commonly 

 received distinction was drawn between the 

 sexual and the non-sexual modes of genera- 

 tion ; and under the latter form a variety of 

 processes of Gemmation and Division were 

 alluded to as occasional or constant substi- 



* A variety of circumstances have contributed 

 to cause delay in the appearance of the present 

 article, some of them of a nature beyond the con- 

 trol of the author. He is sensible, however; that 

 an apology is due by him to the readers of this 

 work on account of the protraction of that delay. 

 He has only to say, that in the contemplation of 

 the vastness and imperfectly known condition of 

 the subject, he has ever felt more disposed to 

 engage in the investigation of some of its details, 

 than to appear before the public as a systematic 

 writer in regard to it. The delay may have this 

 advantage, however, that it will enable him to in- 

 troduce a greater number of new discoveries, a more 

 accurate statement of individual facts, and more 

 correct and extended general views of the subject 

 than might have been possible at an earlier period, 

 and that it will afford him an opportunity of cor- 

 recting and amplifying various statements con- 

 tained in the previous Article GENERATION, which 

 the progress of discovery since the time of its pub- 

 lication has rendered necessary. 



tutes in a certain number of animals for the 

 more permanent sexual form of the reproduc- 

 tive process. At the time of the publication 

 of that article, the sexual organs had not been 

 discovered in a considerable number of the 

 lower animals : but since then, the assiduous 

 and accurate researches of embryologists 

 have gradually diminished the number of 

 animals so situated, by bringing to light the 

 male and female reproductive organs, or their 

 essential products, in nearly every species of 

 the animal kingdom ; so that now only a very 

 few, and those of the simplest organisation, 

 remain, in which the bisexual condition has 

 not been detected. These animals belong 

 exclusively to the division of the animal king- 

 dom recently established by Zoologists, as 

 Protozoa, comprehending the Polygastric In- 

 fusoria, Rhizopoda and Porifera.* 



In all other animals it is now ascertained 

 that fecundated ova, formed by an act of 

 sexual generation, are the means of securing 

 the permanent reproduction of the species ; 

 but in several of them, as is especially well 

 known among the Polypine tribes, a vast 

 multiplication of individuals, sometimes living 

 separately, but more frequently associated in 

 groups, or living in united colonies, takes 

 place by a non-sexual process of reproduction, 

 which may be compared in many instances to 

 the growth or repetition of the parts of a tree 

 or plant by budding. 



Recent investigations have made it more 

 and more apparent, that the non- sexual multi- 

 plication of animals ought to be distinguished 

 into several kinds, according to the different 

 circumstances in which it may occur. In a 

 few, as already remarked, it is entirely with- 

 out known sex : in others, the non-sexual pro- 

 cess of gemmation, or division, gives rise to 

 new individuals, which are simply the repe- 

 titions of the perfect or complete animals ; and 

 in a third set, the non-sexual multiplication 

 occurs more frequently in an incomplete con - 

 dition of the animal, and often consists in the 

 production of one or more series of dissimilar 

 forms of animals, the last generation of which 

 alone becomes sexually complete, and propa- 

 gates the species by fecundated ova. This 

 constitutes the variety of the reproductive 

 process recently distinguished by the name of 

 Alternating Generation. 



Three forms, therefore, of non-sexual animal 

 reproduction, or multiplication, are to be dis- 

 tinguished from the sexual mode of generation, 

 as in the following enumeration : 



I. True sexual generation, direct or indi- 

 rect, in all animals, excepting the Protozoa. 



II. Non-sexual multiplication, occurring 

 only in some of the invertebrated animals ; 



1st. In Protozoa, in which sexual organs 

 have not yet been discovered. 



* The first two of these divisions may be described 

 as simple unicellular microscopic animalcules, the 

 third rather as a compound or congeries of micros- 

 copic animalcules : the Porifera, or Sponges, are in- 

 cluded in this division of Protozoa, because the 

 balance of evidence is decidedly in favour of their 

 animal nature. 



