address. lxxxiii 



germs appear in the parental body, while still embryonic, at a very early period 

 of its development, and clearly derivo their origin from a deeply-seated part 

 of the formative cells which are undergoing transformation into the primitive 

 organs ; but the exact seat of the origin of the two kinds of reproductive 

 cells is still a matter of doubt. 



When the ovum attains its full maturity in the ovary, the scat of its 

 formation within the parent, it is separated from that organ, and when fer- 

 tilized proceeds to undergo embryonic development, differing in this respect 

 from the germinal product of the higher plants, in which the embryo is deve- 

 loped in the place of formation of the seed. 



The period of maturation of the ovum is marked in the greater number 

 of animals by a series of phenomena which have generally been interpreted 

 as the extrusion or absorption of the germinal vesicle ; and various observers 

 have actually traced the steps of the process by which that vesicle appears 

 to leave the yolk and is lost to sight, or has passed into the space between 

 the yolk and its membrane in tho shape of the peculiar hyaline bodies named 

 the polar or directing globules. But recent researches, afterwards to be 

 referred to, tend to show that some part at least of the substance of the ger- 

 minal vesicle remains to form, when combined with the fertilizing element, 

 the newly endowed basis of future development. 



Among the earliest changes to which the perfect animal ovum is subject, 

 I have first to refer to the segmentation of the germ, a series of phenomena 

 the observation of which has been productive of most important results in 

 leading to a comprehension of the intimate nature of the formative process, 

 and which is of the deepest interest both in a morphological and histo- 

 logical point of view. This process, which was first distinctly observed by 

 Prevost and Dumas more than fifty years ago, and is now known to occur in 

 all animal ova, consists essentially in the cleavage or splitting up of the 

 protoplasmic substance of the yolk, by which it becomes rapidly subdivided 

 into smaller and more numerous elements, so as at last to give rise to the 

 production of an organized stratum of cells out of which, by subsequent 

 changes, the embryo is formed. 



The process of yolk-segmentation may at once be distinguished as of two 

 kinds, according as it affects in the small-yolked ova the whole mass of the 

 yolk simultaneously, or in the large-yolked ova is limited to only one part of 

 it. The cleavage process, in fact, affects the germinal and not the food-yolk ; 

 so that to take the two most contrasting instances of the bird and mammal, 

 to which I have before referred, it appears that while the mammal's ovum 

 undergoes entire segmentation, this process is confined to the substance of the 

 cicatricula or germinal disk of the bird's egg. This process is essentially one 

 of cell-division, but it is also in some measure ono of cell-formation. The 

 best idea of its nature will be obtained from a short description of the total 

 segmentation occurring in the mammal's ovum. 



