528 R. JV. Hegner 



free from cytoplasm. The yolk-globules become smaller and 

 smaller posteriorly until they cease altogether in the middle 

 region, where smaller and lighter yolk granules take their place. 

 At the posterior end there are many irregular vacuoles caused by 

 the accumulation of fat in this region. During the hour the egg 

 was under the influence of centrifugal force the preblastodermic 

 nuclei (Fig. 17 pbl. n) migrated outward until they fused with the 

 **Keimhautblastem" forming the blastoderm. The "Keimhaut- 

 blastem" in the mean time flowed away from the anterior end of 

 the egg, adding this portion to that posterior to it and producing a 

 blastoderm in the latter region decidedly thicker than usual. The 

 nuclei in the blastoderm seem to have been influenced by the 

 centrifugal force; those near the central region have apparently 

 been drawn out of their normal spherical shape and are now oval. 

 As the inner pole is approached the nuclei become less and less 

 oval until at the extreme end they are spherical as normally. The 

 vitellophags have migrated toward the axis of rotation and the 

 outer end is free from them altogether, while a greater number 

 than usual are present near the posterior end. The centrifugal 

 force used has apparently had no efi^ect upon the position of the 

 nuclei of the vitellophags in relation to the mass of cytoplasm 

 which surrounds them, as in every case the nucleus is in or near 

 the center. The direction of division of these vitellophags, how- 

 ever, seems to have been influenced for we find in almost every 

 instance that the daughter cells produced by a recent division lie 

 one posterior to the other, i.e..^ in the direction of the centrifugal 

 force. The germ-cell determinants have found their way as 

 usual into the primordial germ-cells at the extreme posterior end 

 of the egg (Fig. 1%, p. gc). 



C.B. 2, c. A normal embryo (Fig. 7) was produced by this 

 egg, which was fixed forty-eight hours after being centrifuged. 

 Not the slightest diff'erence could be discovered between an in toto 

 preparation of this egg and a normally developed egg of the same 

 age (63 hours). Sagittal sections show that the yolk has under- 

 gone segmentation and that the yolk-spheres and }/olk-granules are 

 equally distributed throughout the entire yolk mass. The germ- 

 cells have migrated from the posterior amniotic cavity through the 



