180 NUCLEOCYTOPLASMIC RELATIONS 



sac. This seems more a fortuitous circumstance than an actual 

 requirement, however, since eggs will occasionally emerge an- 

 terior end first; several have been closely observed and all de- 

 veloped normally. Much of the cytoplasm is distributed in a 

 layer 5 to 10 microns thick around the egg periphery, and the rest 

 is threaded through the yolk that constitutes the major portion 

 of the egg. The nucleus of the unlaid egg lies in the egg interior 

 at the anterior end (see Fig. 5). When oviposition takes place, 

 the egg is tightly squeezed through the ovipositor and emerges 

 with the nucleus now lying against the convex (ventral) surface 

 at the anterior end. It has been suggested that the act of squeez- 

 ing displaces the nucleus and activates the egg into resumption 

 of normal development. The egg obviously is not activated by 

 sperm since fertilization is not necessary for normal development. 

 The mechanical interpretation of egg activation is merely a re- 

 cording of observed events in sequence. It would seem that until 

 eggs can be removed from uterine sacs and artificially activated 

 the activation process will not be fully understood. 



After oviposition the first meiotic division begins and the 

 chromosomes go through a brief interphase, which is quickly 

 followed by the second division. The divisions leave the four 

 meiotic nuclei in a linear quartet perpendicular to the egg sur- 

 face, all within the egg. The outer polar nucleus quickly disinte- 

 grates, the two middle polar nuclei fuse and form an abortive 

 metaphase plate, and the inner nucleus becomes the functional 

 pronucleus. At 30°, meiosis is completed in thirty minutes after 

 oviposition. 



The pronucleus migrates into the egg interior, and nuclear 

 cleavages quickly begin. If the egg is fertilized, the sperm nu- 

 cleus comes to lie beside the egg nucleus and forms a double 

 metaphase plate, a characteristic not observed in later cleavages. 

 Nuclear cleavage is a rapid, orderly, and at first completely syn- 

 chronous process. During later cleavages, nuclei in the anterior 

 end divide somewhat sooner than nuclei in the posterior end. 

 This paitial asynchrony is not an artifact, since it is observed 

 whether fixation is done at 50°, 2°, or —83°, or whether either 

 end is punctured before fixation. At the tenth cleavage, when 



