S86 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



developmental pattern in the remainder of the egg may show no funda- 

 mental alteration (Hogue, 1910). With stronger centrifuging continued 

 from the stage of separate pronuclei through the first cleavage, the eggs 

 are flattened by the force, the first cleavage spindle lies at right angles 

 to direction of the force, and the first cleavage plane coincides with direc- 

 tion of force. In many of these eggs a cytoplasmic ball containing the 

 heavier granules separates at the centrifugal pole during the first cleavage 

 (Fig. 183) because the cleavage furrow does not penetrate this dense region 

 (Boveri, 1910a, b; Hogue, 1910). Both cells resulting from first cleavage 

 of these eggs behave like the ventral cell, P^ (Fig. 179), as regards absence 

 of chromatin diminution and later cleavage pattern. According to Boveri 



(1910&), occurrence or nonoccurrence of 

 chromatin diminution depends on level of 

 the cytoplasmic gradient at which nuclei 

 lie. In the "ball eggs" both nuclei result- 

 ing from first cleavage are at the same 

 level; consequently, they behave in the 

 same way, and the level is evidently suffi- 

 ciently basal so that diminution does not 

 take place. Since the two blastomeres of 

 these eggs include the whole, or almost 

 the whole, polar gradient, their cytoplas- 

 mically determined nuclear condition may 

 play a part in determining their behavior as ventral cells. Ultracentrifug- 

 ing Ascaris eggs usually suppresses the first cleavage, but nuclear division 

 continues, and all nuclei usually undergo diminution (King and Beams, 

 1938). To account for these results a specific chemical diminisher sub- 

 stance is postulated, formed from cytoplasmic substance, more concen- 

 trated apically, and produced slowly, so that normally the first cleavage 

 occurs before it diffuses throughout the egg. With suppression of first 

 cleavage it diffuses more or less throughout; consequently, with few ex- 

 ceptions, all nuclei undergo diminution. At present the hypothesis that 

 diminution may be a reaction to nonspecific differences in metabolism 

 seems equally plausible. 



Eggs of Cyclops apparently do not orient in the centrifuge but become 

 stratified and elongate in direction of the force. The first cleavage spindle 

 is parallel to the stratification; consequently, the first cleavage is in the 

 greatest diameter of the egg rather than in the smallest, as normally, but 



Fig. 183. — Separation of a cyto- 

 plasmic ball at first cleavage of strong- 

 ly centrifuged Ascaris eggs (after Bo- 

 veri, 1910a). 



