34 



THE BIOLOGY OF TWINS 



If the site of the germinal vesicle is that of the animal 

 pole, the vegetative pole is that which is most nearly 

 in contact with the yolk mass.' 



During maturation an extremely radical change in 

 polarity and general organization takes place. The 



comparatively homo- 

 geneous forma-tive 

 zone of protoplasm 

 moves to one pole of 

 the egg and forms a 

 cap with a crescentic 

 cross-section, thick 

 in the middle and 

 thinned out at the 

 edges (Fig. 6). The 

 originally central 

 yolk mass moves to 

 the pole of the egg 

 opposite to that occu- 

 pied by the formative 

 cap, and comes to lie 

 in contact with the 

 zona pellucida for 

 about one-third of the periphery of the egg. The germi- 

 nal vesicle during this reorganization process enters the 

 stage of a first polar spindle, which, pecuUarly enough, 

 lies tangentially to the periphery of the ovocyte and very 

 close to the boundary between the formative and deu- 



dz 



f 



Fig. 6. — Maturating egg of the arma- 

 dillo, showing total rearrangement of 

 materials. The yolk (i z) with yolk 

 granules {i g) occupies the animal pole, 

 and the formative protoplasm (/ z) occu- 

 pies the opposite pole in the form of a 

 cap. The nucleus {_p s) is dividing to 

 form the first polar body and lies tan- 

 gentially to the periphery. 



■ This central position of the yolk and the peripheral position of 

 the formative protoplasm are strikingly like those described by HUl 

 (igio) for the marsupial Dasyurus and may therefore be interpreted 

 as a primitive character. 



