SEGMENTATION OF THE OVUM. 



55 



divided into two cells, the polar globules lie in the angle between these two cells 

 (Fig. 8), because there the globules find room. It is to be noted that the glo- 

 bules accommodate themselves to the segmentation spheres, and that the forma- 

 tion of the spheres is not accommodated to the original position of the 

 globules. 



The degree of the eccentricity of the segmentation spindle varies in different 

 ova, chiefly according to the amount of yolk ; the greater the quantity of yolk 

 in the ovum, the more marked is the eccentricity. 



The actual first cell division (first cleavage or first segmentation) of a mam- 

 malian ovum has never been followed by direct observation, the practical diffi- 

 culties not having hitherto been successfully overcome. Various phases of the 



FIG. 8. OVUM OF A RABBIT OF TWENTY-FOUR FIG. 9. OVUM OF A SNAIL (LIMAX CAMPESTRIS) 

 HOURS. DURING THE FIRST CLEAVAGE. THE ENVEL- 



The first cleavage has been completed ; the two cells OPES OF THE OVUM ARE NOT DRAWN IN 



(segmentation spheres) are appressed; above the X 200 diams. (After E. L. Mark.} 



cells lie the polar globules ; numerous spermato- 

 zoa lie in and within the zona pellucida. (After 

 Coste. ) 



division have, however, been seen and the internal changes have been studied by 

 means of sections. It accordingly seems expedient to interpolate the following 

 account of the external appearances of the first segmentation in the living ovum 

 of the snail, Limax campestris. The eggs of this animal, by their size and in 

 their mode of segmentation, have a certain resemblance with mammalian ova. 

 The following description is taken from the account by E. L. Mark, published in 

 1 88 1 ; it is nearly in his own words : 



In Limax, after impregnation, the region of the segmentation nucleus 

 remains more clear, but all that can be distinguished is a more or less circular, 

 ill-defined area, which is less opaque than the surrounding portions of the vitel- 

 lus. After a few moments this area grows less distinct. It finally appears 



