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TEE POPULAR SCIENCE MONTELT.— SUPPLEMENT. 



peculiar hyaline bodies named the polar or direct- 

 ing globules. But recent researches, afterward 

 to be referred to, tend to show that some part, at 

 least, of the substance of the germinal vesicle 

 remains to form, when combined with the fertil- 

 izing element, the newly-endowed basis of future 

 development. 



Among the earliest changes to which the per- 

 fect animal ovum is subject, I have first to refer 

 to the segmentation of the germ, a series of phe- 

 nomena the observation of which has been pro- 

 ductive of most important results in leading to a 

 comprehension of the intimate nature of the for- 

 mative process, and which is of the deepest in- 

 terest both in a morphological and histological 

 point of view. This process, which was first dis- 

 tinctly 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 organ- 

 ized 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-yolkcd 

 ova is limited to only one part of it. The cleav- 

 age process, in fact, affects the germinal and not 

 the food yolk ; so that, to take the two most con- 

 trasting instances of the bird and mammal to 

 which I have before referred, it appears that 

 while the mammal's ovum undergoes entire seg- 

 mentation, this process is confined to the sub- 

 stance 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 one 

 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. 



When, as before mentioned, the germinal vesi- 

 cle has been in part extruded or lost to sight, the 

 whole yolk-substance of the ovum forms a nearly 

 uniform mass of finely granular protoplasm, in- 

 closed within the external cell-membrane. With- 

 in a few hours later a clear nucleus has arisen in 

 this mass. To this more definite form of organi- 

 zation, assumed by the germinal substance of the 

 future animal which is about to be the subject 

 of the segmenting process, the name of the first 

 segment-sphere may be given. 



By the process of cleavage, which now begins, 



this first segment-sphere and its nucleus undergo 

 division into two nucleated spheres of smaller 

 size, the whole substance of the yolk, in a holo- 

 blastic ovum, such as that of the mammal, being 

 involved in the segmenting process. 



The second stage of division follows after the 

 lapse of a few hours, and results in the formation 

 of four nucleated segment-spheres ; and the proc- 

 ess of division being repeated in a certain defi- 

 nite order, there result in the succeeding stages, 

 that is, the third, fourth, fifth, and up to the tenth, 

 the numbers of 8, 12, 16, 24, 32, 48, 64, and 96 

 nucleated yolk-spheres, germ-spheres, or forma- 

 tive cells. 



In the rabbit's ovum the tenth stage is reached 

 in less than three days ; and as during that time 

 the size of the whole ovum has undergone very 

 little increase, it follows that the spheres of each 

 succeeding set, as they become more numerous, 

 have diminished greatly in size. These segment- 

 spheres are all destitute of external membrane, 

 but are distinctly nucleated ; and their protoplas- 

 mic substance is more or less granular, present- 

 ing the usual histological characters of growing 

 cells. 



By the time that segmentation has reached 

 the seventh or eighth stage, when thirty-two or 

 forty-eight spheres have been formed, the ovum 

 has assumed the appearance of a mulberry, in 

 which the outer smaller spheres, closely massed 

 together, project slightly and uniformly over the 

 whole surface ; while the interior of the ball is 

 filled with cells of a somewhat larger size and a 

 more opaque granular aspect, also resulting from 

 the process of segmentation. 



Already, however, the mutual compression of 

 the spheres or cells on the surface, by their 

 crowding together, has led to the flattening of 

 their adjacent sides; and by the time the tenth 

 stage is reached, when the whole number of the 

 cells is about ninety-six, the more advanced super- 

 ficial cells, having ranged themselves closely to- 

 gether, form a neucleated cellular layer or cover- 

 ing of the yolk, inclosing within them the larger 

 and more opaque cells, derived like the first from 

 the segmenting process. In a more advanced 

 stage, the deeper cells now referred to having also 

 taken the form of a layer, there results at last 

 the bilaminar blastoderm or embryonic germinal 

 membrane. 



The process of partial segmentation, such as 

 occurs in the bird's egg, though perhaps funda- 

 mentally the same as that of the mammal pre- 

 viously described, stands in a different relation 

 to the parts of the whole yolk or egg, and conse- 



