OVUM. 



cell appears with great probability to take 

 its origin from the germ-cell, or its nu- 

 cleus, or from some part of it, in combination 

 with a determinate portion of the sperm 

 product, or descendent of the sperm-cell ; and 

 we are so far justified, therefore, in ascribing 

 the genetic process by which the new being 

 is formed to the mutual action of the products 

 of two different kinds of cells, viz., the germ- 

 cell and the sperm-cell. * 



In conclusion, the ovum may be defined to 

 be a distinct vesicular body originally formed 

 from a ce 1, presenting throughout its exist- 

 ence the organic cellular structure, consisting 

 of oleo-albuminous materials, formed by the 

 female of an animal species, and capable, when 

 acted on by the spermatic product of the 

 male, of undergoing the successive changes of 

 embryo-genesis, by which, either directly or 

 through intervening generations, the species of 

 animals is reproduced and continued. 



The structural distinctive characters of an 

 ovum are, therefore, its enclosure within a 

 distinct vesicular covering, and its original 

 organic cellular constitution in the germ-cell : 

 its most important physiological characteristic 

 is its susceptibility of the changes of embry- 

 onic development under the influence of the 

 sperm-cell or its product. 



II. OF THE NON-SEXUAL MODE or GENE- 

 RATION. 



The necessity of distinguishing several kinds 

 of non-sexual reproduction according to its 

 occurrence in animals entirely without sex, or 

 believed to be so, and in those which mav 

 also be propagated in the sexual mode, has 

 already been adverted to. A farther distinc- 

 tion of the non-sexual reproduction may be 

 made according to the nature of the process 

 itself: thus, some forms of it consist in the 

 development of buds, so intimately united 

 with the parent substance, that scarcely any 

 difference can be perceived between their 

 mode of formation and that of continuous 

 growth, as in Hydra and various Polypes : 

 other forms consist in the development of 

 new individuals from germs so isolated in 

 their form and cellular in their structure, 

 that it might seem at first sight arbitrary 

 to distinguish them from ova, as in Aphides ; 

 others appear to hold an intermediate place 

 and character between these two forms, as in 

 ISalpa: while, in a fourth set, a more complex 

 and varied series of changes occurs, which may 

 be regarded with probability as modifications 

 of the gemmal or germinal processes, as in 

 Medusoid Polypi, Taenia, &c. But it will be 

 apparent from what follows that we are as 

 yet very far from that exact knowledge of the 

 nature and first origin of buds, gemmae, or 

 other kinds of germs, from which animals may 

 be multiplied in the non-sexual modes, which 

 would enable us to form satisfactory general 



* These views have been stated with great clear- 

 ness by Prof. Owen in his various writings, especi- 

 ally in his Essay on Parthenogenesis, and Lectures 

 on Generation, &c., in Medical Times, 1849, and by 

 Dr. Carpenter in his Principles of Physiology, Ge- 

 neral and Comparative. 1851. 



conclusions as to their mutual relations, and 

 their similarity or difference, as compared on 

 the one hand with organic growth, and on the 

 other with oval development. 



As the accurate determination of these re- 

 lations is in a great measure impossible, it will 

 be expedient for the present to state only 

 very briefly the general characters of the 

 several non-sexual modes of reproduction, 

 before selecting for more particular consider- 

 ation some varieties of the process, the recent 

 investigation of which seems calculated to 

 influence in a considerable degree future ge- 

 neral views of the whole subject of reproduc- 

 tion. We shall also defer for the present any 

 minute consideration of the relation of these 

 processes to the growth or development of 

 cells, for we shall have occasion to treat more 

 at length of that subject in a subsequent part 

 of this article, and in that of vegetable ovum.* 



At this place it is only necessary to re- 

 mind the reader, that all processes of develop- 

 ment, whether in the earliest or at more ad- 

 vanced stages of formation, appear to consist 

 essentially in, or are more or less intimately 

 connected with, a multiplication of organic 

 cells in the parts that are developed. In the 

 unicellular beings, fissiparous and gemmiparous 

 multiplication may easily be recognised to be 

 processes of cell growth ; the one consisting 

 in the division of the parent cell into a pro- 

 geny of two by a nearly equal partition of its 

 substance ; the other, in an extension and 

 gradual enlargement of a small or limited por- 

 tion of the original cell. But in many of the 

 instances of fission and gemmation on the larger 

 scale with which we are acquainted, observa- 

 tion has not yet pointed out the primary cell, 

 if it exists, from which the process of division 

 or extension begins jand,indeed, most instances 

 of fissiparous division may. as Dr. Carpenter 

 has remarked, be referred to a peculiar modifi- 

 cation of gemmation. 



The process of budding or gemmation is 

 usually stated to occur in one of two modes. 

 1st, by the extension of a part of the parent 

 body which remains in organic connection 

 with it during the development of the new 

 individual from the bud ; the attached bud 

 either sprouting from the exterior, or being 

 developed in the interior of the parent stock. 

 2nd, by the development of the new individual 

 from a small detached portion of the substance 

 of the parent, which undergoes the principal 

 formative changes after its separation. These 

 separate buds have been called gemmae, gem- 

 mules, bulbils, &c., and two kinds of them 

 may also be distinguished according as they 

 are thrown off from the external surface of 

 the parent body, or are formed and become 

 loose within its interior. These gemmules 

 have frequently attained to some degree of 

 development by the time of their separation, 

 and very often are provided with cilia over 

 their surface, which cause them to move 



* For a very lucid and agreeable statement of 

 these relations the reader is referred to Dr. Carpen- 

 ter's able Treatise on General and Comparative 

 Phys'.ology. 1851. 



B 3 



