36 THE MATURATION OF THE EGG OF THE MOUSE. 



the number of the circumpolar bodies, and often by their complete 

 disappearance, and by the disappearance of the clear region previously 

 described as surrounding the first spindle. 



Stage IX. — Division of Second Maturation Spindle. 

 (Plate 5, figs. 28 to 30.) 



The separation of the daughter chromosomes takes place, as a rule, 

 only after a spermatozoon has touched or penetrated the egg. How- 

 ever, in the case of one animal — a mouse which had not been insemi- 

 nated — one of the eggs contained the divided chromosomes arranged 

 in two parallel daughter plates, which were still near the equator of the 

 spindle ; another egg from the same mouse presented a stage still further 

 advanced (plate 5, fig. 28), the two groups of daughter chromosomes 

 in this case having migrated nearer to the poles of the spindle. 



Stage X. — Telophase of Second Spindle and Second Polar Cell. 



(Plate 5, fig. 30.) 



The beginning of the abstriction of the second polar cell resembles 

 that of the first. This stage, indeed, agrees so closely with the corre- 

 sponding stage in the formation of the first polar cell (Stage VI, p. 27), 

 from which it seems to differ only in the presence in the oocyte of the 

 head of a spermatozoon, that it need not be described here. It may be 

 said that, of 30 eggs in this stage, only 1 failed to show the head of a 

 spermatozoon. 



2. CHROMATIN PARTS OF SECOND MATURATION SPINDLE. 



The chromosomes of the second maturation spindle arise directly 

 from the chromatin mass which remains in the egg after the abstric- 

 tion of the first polar cell, i.e., without an intervening vesicular stage 

 of the nucleus. This mass breaks up into fragments, but whether or 

 not each of these fragments is the equivalent of a chromosome, either 

 single or multiple, it is difficult to determine. Whatever their mode of 

 origin, the fragments are fairly (or even very) irregular in form, incom- 

 pletely separated from one another, and of uncertain number (plate 4, 

 fig. 19). Some of them bear a slight resemblance to the daughter chro- 

 mosomes of the previous division which had nearly reached the poles of 

 the first spindle (fig. 16) . Sometimes (fig. 19b) they are embedded in a ma- 

 trix of homogeneous substance denser than the surrounding cytoplasm. 

 They are never scattered, and soon become arranged in the plane of the 

 equator of the future spindle, where they may constitute a group having 

 the form of an imperfect ring. No stages between this and that of the 

 completely formed chromosomes have been observed. 



The chromosomes of the completed second spindle (figs. 23 and 24) 

 often closely resemble the daughter chromosomes of the first spindle 

 as they appear when they have nearly finished their poleward migra- 

 tion (fig. 16), for each mother chromosome of the second spindle is com- 

 posed of a pair of elements, and these elements vary in form, independ- 



