42 



THE EARLY DEVELOPMENT OF MAMMALS. 



Segmentation of the Ovum. 



Shortly after the entrance of the spermatozoon into the ovum the segmenta- 

 tion spindle is developed by the union of the pro-nuclei, as described in the pre- 

 vious section. This spindle leads to a division of the ovum into two cells. These 

 cells further rapidly divide. As stated on page 10, these early cell divisions are 

 called the segmentation of the ovum. 



The position of the first segmentation spindle is always eccentric, and appears 

 to be approximately, if not exactly, the same as that of the egg-cell nucleus before 

 maturation. The axis of the spindle varies greatly in its direction. It is some- 

 times at right angles to the radius of the ovum, which passes through the polar 



FIG. g. OVUM OF A RABBIT OF TWENTY-FOUR FIG. 7. OVUM OF A SNAIL (LIMAX CAMPESTRIS) 



DURING THE FIRST CLEAVAGE. THE ENVEL- 

 OPES OF THE OVUM ARE NOT DRAWN IN. X 2OO 



diams. (After E. L. Mark.) 



HOURS. 



The first cleavage has been completed; the two cells 

 (segmentation spheres) are appressed; above the 

 cells lie the polar globules; numerous spermato- 

 zoa lie in and within the zona pellucida. 

 (After Coste.) 



globules, but it is more usually oblique to this radius. It was at one time thought 

 that the plane of division was always at right angles to the radius of the extrusion 

 of the polar globules, but this view cannot be upheld. After the ovum has divided 

 into two cells, the polar globules lie in the angle between these two cells (Fig. 6), 

 because there the globules find room. It is to be noted that the globules accom- 

 modate themselves to the segmentation spheres, and that the formation 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 di- 

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

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



