56 



THE EARLY DEVELOPMENT OF MAMMALS. 



elongated. Very soon this lengthening results in two light spots, which are in- 

 conspicuous at first, but which increase in size and distinctness, and presently 

 become oval. If the outline of the egg be carefully watched, it is now seen to 

 lengthen gradually in a direction corresponding to the line which joins the spots. 

 As the latter enlarges the lengthening of the ovum increases, though not very 

 conspicuously. Soon a slight flattening of the surface appears just under the 

 polar globules; the flattening changes to a depression (Fig. 9), which grows 

 deeper and becomes angular. A little later the furrow is seen to have extended 

 around on the sides of the yolk as a shallow depression, reaching something more 

 than half-way toward the vegetable or inferior pole, and in four or five minutes 



FIG. 10. OVUM OF WHITE MOUSE. FIRST SEG- 

 MENTATION SPINDLE WITH EQUATORIAL PLATE 

 OF CHROMOSOMES. X I 5 diams. (After 

 Sobotta. ) 



FIG. ii. OVUM OF WHITE MOUSE. FIRST SEG- 

 MENTATION SPINDLE. 



The chromosomes have divided and have migrated 

 toward the poles of the spindle, forming two 

 groups. X 1500 diams. (After Sobotta.} 



after its appearance the depression extends completely around the yolk. This 

 annular constriction now deepens on all sides, but most rapidly at the animal 

 pole; as it deepens it becomes narrower, almost a fissure. By the further deep- 

 ening of the constriction on all sides there are formed two equal masses connected 

 by only a slender thread of protoplasm, situated nearer the vegetative than the 

 animal pole, and which soon becomes more attenuated and finally parts. The 

 first cleavage is now accomplished. Both segments undergo changes of form; 

 they approach and flatten out against each other, and after a certain time them- 

 selves divide. 



The Internal Changes in the Mammalian Ovum. At the close of impregna- 

 tion the segmentation spindle is completely formed (Fig. 10). The chromosomes 



