90 



MANUAL OF HISTOLOGY. 



Fig. 89. Segmentation of the mammalian ovum (half- 

 diagrammatic). 1, Yelk cleft in two ; 2, further subdivided 

 into four spherules (cells; with nuclei ; 3, a large number 

 of nucleated cells; 4, a b, separate -corpuscles. 



place in the mammalia is, unfortunately, not yet conclusively ascer- 

 tained. The primordial nucleus of the ovum, however, known as the 



" germinal vesicle, seems first 

 to disappear ; after this two 

 transparent spots are seen, 

 two new nuclei, and around 

 each, half of the cell-body or 

 yelk, by which name the latter 

 is known here (1). By further 

 subdivision, four cells are 

 formed from these two seg- 

 ments (2) ; and from these 

 again eight, and so on, until 

 finally, in consequence of re- 

 peated segmentation, the cap- 

 sule of the ovum contains a 

 multitude of small nucleated 

 cells (3, 4). From the latter 

 the first rudiments of the 

 embryonic body are formed : 

 from them spring all normal 

 and pathological form-ele- 

 ments ; they are the most 

 important and highly destined 

 cells in the whole system. 

 Throughout the whole animal kingdom this segmentation of cells is 

 observed in the ovum. Those cases are particularly instructive in which 

 the original nucleus of the egg (germinal vesicle) (seen among some low 

 groups of animals) is found to remain, and in which the phenomena of 

 segmentation may then be followed up with the greatest ease on the 

 nuclei with distinct nucleoli, which have taken their origin from it. It 

 is to be hoped that further research may lead to the same results in regard 

 to the mammalian egg, and thus rid the theories of yelk-segmentation of 

 many contradictions and difficulties which at present offer such unpleasant 

 obstacles to true progress. 



As regards the mechanism of the process of segmentation, science is not 

 yet able to give any satisfactory explanation. There can be no doubt, 

 however, that the vital contractility of the cell-body plays an important 

 part in it ; for only in young elements, i.e., containing protoplasm, do we 

 observe the process of multiplication to take place. "Were it the case that 

 both cell and nucleus were always similarly affected by the act, we might 

 suppose the latter to be simply divided passively by the constriction of 

 the protoplasm. But this is contraindicated by the occurrence of two 

 nucleoli in a still simple nucleus, as also of two nuclei widely separated 

 from one another in a cell-body which has as yet undergone no change 

 (fig. 86 c). 



One fact, adduced from extended observation, is of great importance, 

 namely, that the whole process of division may be, and usually is, corn- 

 completed very rapidly, probably within the space of a few minutes. This 

 enables us to comprehend the enormous proliferation of cells which we not 

 unfrequently meet with in pathological processes. It also explains the 

 fact that cells engaged in the act of segmentation are comparatively rarely 

 met with, even where the liveliest plastic processes are going on in an organ. 



