36 THE BIOLOGY OF TWINS 



Prior to the complete abstriction of the yolk mass 

 the egg completes its nuclear maturation. Studies 

 of the chromosomes show that there are thirty-two 

 in the ovocyte of the first order, and that after tj^ical 

 tetrad formation the number is reduced to sixteen in 

 the first polar body and to sixteen in the ovocyte of 

 the second order. The second maturation division is 

 equational and produces a second polar body. It is 

 very rare, however, to find a second polar body formed 

 before the process of ovulation. This process probably 

 occurs while the egg is in the oviduct just before fertiliza- 

 tion. After a long search for tubal ova, I was fortunate 

 enough to find one fertilized egg, apparently quite 

 normal in every respect, in a part of the oviduct not 

 far from the fimbriated funnel. This egg showed just 

 two polar bodies and the male and female pronuclei 

 l}dng close together in the formative protoplasm (Fig. 7) . 

 The deutoplasm had not yet been extruded. From this 

 we might infer that yolk elimination occurs as an 

 accompaniment of the first cleavage division, as in 

 Dasyurus. A study of parthenogenetic cleavage' shows 

 numerous stages in which the small yolkless egg hes 

 within the zona pellucida surrounded by fragments of 

 yolk. Later stages show very pretty cleavage spindles 

 in the egg; clean-cut four-cell and somewhat doubtful 

 eight-cell stages have been found. This is apparently 

 as far as parthenogenetic development goes, so we 

 must await the discovery of the events in normal 

 cleavage before we can know with certainty what form 

 of cleavage we have in the armadillo: whether it is 

 regular, as in Dasyurus, or indeterminate, as in the 



' H. H. Newman, Biological Bulletin, XXV (1913). 



