OVUM. 



and almost always exhibiting, when recently 

 mixed with water, vivid vibratory or umlula- 

 tory movements, but in a few animals present- 

 ing other forms, and without motion. These 

 spermatic filaments or particles are developed 

 by a peculiar process in the interior of the 

 cells (tpeni-cellt) secreted in the male organ 

 or testis. 



When the ovarian ovum has arrived at 

 maturity, the germ cell disappears as such, 

 and if fecundation shall have taken place, that 

 vesicle is succeeded by another minute cell, 

 with which the origin and development of the 

 new animal are most intimately associated. 

 This secondary organic cell of the fecundated 

 ovum has therefore been called the Embri/o- 

 ccll. The first changes, preparatory to the 

 commencement of the development of an 

 embryo, consist in the formation out of the 

 embryo-cell and yolk substance of an organised 

 cellular mass, or of a membranous covering of 

 the whole or a part of the yolk: this is the 

 germ-mass, Blastoderm, or germinal membrane. 



Fig. 3. 



cli'o 



Fecundated Ovum of a Mammifer, li'ith the Embryo 

 Cell and its division. 



a, ovum with the first embryo-cell ; &, division 

 of embryo-cell and cleavage of the yolk round it ; 

 c, second division and cleavage ; d, farther division ; 

 and e, germ-mass or Blastoderm funning; f, dia- 

 gram of the embryo with its membranes, the am- 

 nion, allantois, &c., within the chorion. 



The process by which this primary organised 

 part is produced varies somewhat in different 

 animals; bat it appears to consist in a mul- 

 tiplication of the embryo-cell b}' changes 

 of the nature of cytogenesis, accompanied 

 with more or less of a cleavage or sub-division 

 of the substance of the yolk, and its com- 

 bination with the progeny of the embryo-cell. 

 The general result is, that the first rudiments 

 of the new being take their origin in organic 



cells, which are descended from the original 

 embryo-cell. 



From this blastodermic mass or membrane, 

 the embryo, or foetus, or new animal, and in 

 the higher animals some accessory parts, 

 which are temporarily united with the embryo 

 previous to its birth, originate, and are gra- 

 dually formed, by a various process of pro- 

 gressive organic growth of an epigenetic 

 character, which is termed Development, or 

 Embryo-genesis. 



In by far the greater number of animals an 

 ovum gives rise to only one embryo or indi- 

 vidual, and this one becomes by itself, when its 

 growth is complete, the perfect sexual animal, 

 capable of contributing its share to the pro- 

 duction of fecundated ova. But in a certain 

 number of animals, to which allusion will be 

 made more fully afterwards, the immediate 

 product of development from the ovum is not 

 at once, and by itself, converted into a com- 

 plete sexual individual ; but by an intermediate 

 non-sexual process of production, one or more 

 new individuals are formed out of the body of 

 that first developed, and to the last so formed 

 is committed the office of sexual reproduction, 

 or true generation. 



The essential conditions and phenomena, 

 therefore, of the sexual process of generation, 

 as related to the ovum, and as limited by the 

 foregoing considerations, may be shortly stated 

 to be the following. 



1st. The formation of the ovarian ovum of 

 the female sex, containing the germ-cell. 



2nd. The formation of the sperm-cells of 

 the male sex, and the development of their 

 peculiar spermatic elements. 



3rd. The mutual action of these two pro- 

 ducts in the fecundation of the ovum. 



4th. The disappearance of the germ-cell of 

 the ovarian ovum, and the formation of the 

 embryo-cell in the fecundated egg. 



5th. The multiplication of the embryo-cell 

 by cytogenesis, and the formation from that 

 body, and from the yolk, or a part of it, of the 

 blastodermic mass or membrane. 



Gth. The process of embryo-genesis, or 

 development of the systems, organs, and 

 textures of the new animal. 



It is right to state that the original germ- 

 cell has not yet been ascertained to exist in 

 the ovum of every animal, nor has its successor, 

 the embryo-cell, been observed in all instances ; 

 but they have been detected in so very large 

 a proportion, that it appears extremely pro- 

 bable that in all sexual animals the generative 

 process consists in the process above described, 

 or in some modification of it. I refrain at 

 present from farther details as to these phe- 

 nomena, and have stated the results only in 

 their most general form, because I shall have 

 occasion to return upon some of them in a 

 subsequent part of the article. Looking 

 back on this general statement of the com- 

 mencement and progress of the genetic 

 process in animals, it will be seen that the 

 new being may be considered as taking 

 its immediate origin from the progeny of 

 cells descended from the embryo-cell. That 



