14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



It has been stated above that the majority of the cleavage nuclei 

 migrate to the surface of the egg. The relatively few that remain 

 behind in the vitellus are destined to produce the primary trophonuclei. 



The migrating cleavage nuclei continue to rise toward the surface 

 of the egg but undergo another cleavage division, the seventh, before 

 they reach the periplasm. In flea eggs it is at the 128 nucleus stage, 

 therefore, that the cortical layer is first supplied with nuclei. Mention 

 should be made of the fact that the mitotic spindles of this seventh 

 division are parallel to the egg's surface and, as a consequence, the 

 daughter nuclei retain the spheroidal arrangement already described 

 for their parent nuclei. Wheeler (1889) f or Blatla, Heymons (1895) 

 for Forficula, and Lecaillon (1897) for Clytra, have described a 

 similar orientation for the mitotic spindles of the cleavage division just 

 prior to the arrival of the nuclei in the periplasm. 



There is a great deal of variation among insect eggs as to the exact 

 region of the periplasm at which the cleavage nuclei first reach the 

 surface. In Pieris (Bobretzky, 1878) and Chironomus (Weismann, 

 1882) the first nuclei arrive in the cortical protoplasm at the anterior 

 pole. In Formica (Ganin, 1869) and Musca (Graber, 1889) the sur- 

 face of the egg is attained first at the posterior pole. Auten (1934) 

 says that in Phormia the nuclei arrive at the two poles slightly before 

 they reach the other surfaces of the egg. In Hydrophilus (Heider, 

 1889), Platygaster (Kulagin, 1897), and Calendra (Wray, 1937) the 

 surface of the egg first reached is between the poles in the equatorial 

 region. In certain other insects such as Blatta (Wheeler, 1889) and 

 Apis (Nelson, 1915) the ventral surface somewhat anterior to the 

 equator is the region first reached. In Callopteryx (Brandt, 1869) the 

 nuclei are said to arrive at the surface in groups. In the development 

 of flea eggs the arrival of the nuclei at the surface does not correspond 

 to any of the above cases for, as a rule, in pulicid ova the nuclei arrive 

 at the periplasm simultaneously at all points. This is not surprising 

 in view of the regular symmetry of these eggs and the fact that the 

 nuclei approach the surface arranged in the form of a hollow, single- 

 layered figure, the outline of which conforms very closely to the shape 

 of the egg surface. The only exception to the simultaneous arrival of 

 the nuclei at the surface is that in some eggs the most posterior nuclei 

 arrive at the periphery a little later than the others. This is apparently 

 due to the fact that the zygotic nucleus sometimes begins dividing at a 

 point slightly anterior to the middle of the egg. As these most posterior 

 cells tend to become germ cells, their further history is postponed for 

 later consideration. 



