EXPERIMENTAL EMBRYOLOGY 151 



of the order Neuroptera, a form not previously studied, is nearer the 

 determinate than the indeterminate type. 



In the beetle Bruchus, Brauer and Taylor (1934) show by constric- 

 tion and cauterization that determination occurs quite early, the egg 

 becoming a true mosaic during blastoderm formation. In Sitona, Reith 

 (1935) reports that the determination process occurs slightly later than in 

 the ant and seemingly at about the same time as in the honeybee. 



Determinate eggs have been found only in the Diptera; but at least 

 as far as the germ cells are concerned, Hegner's results indicate that the 

 chrj^somelid beetles might also be placed here as well as the other insects 

 in which the presumptive germ cells are segregated as "pole cells" during 

 blastoderm formation. Determinate eggs always give mosaic formations 

 under experimental conditions, the few slight exceptions showing an 

 extremely small amount of regulative power. Accordingly, the parts of 

 the embryo must be looked on as entirely self-differentiating after 

 fertihzation or, at least, incapable of development beyond their prospec- 

 tive significance. 



Reith (1925), Pauh (1927), and Rowland and Robertson (1934) 

 report that cauterization of dipterous eggs invariably gives mosaic 

 development. Slight regulative power is shown by Reith's report that 

 the mid-gut anlagen of either end may form an entire mid-gut and also, 

 by Strasburger's (1934) report, that slight cauterizations at the begin- 

 ning of cleavage may delay the migration of the cleavage nuclei to the 

 posterior part of the egg without preventing formation of a normal 

 embryo — the posterior parts developing slightly later than the anterior 

 parts. 



C5rtoplasmic Deterinination.— It is evident that the distinction 

 between "regulative" and "mosaic" eggs in insects, as in other groups, 

 is not sharp and that the designations mean little unless the period of 

 development is specified. The earlier an egg assumes a mosaic nature 

 the "more determinate" it is, but even the most "indeterminate" of 

 insect eggs eventually becomes "determinate." The primary seat of 

 the chemo-differentiation (predetermination) is the cytoplasm as a whole, 

 and the formation of cells plays only a secondary or indirect part. The 

 fact that development is less disturbed by late than by early centrifuging 

 (Hegner, 1909; PauH, 1927; Reith, 19326) suggests that fixation of 

 determination is accompanied by increased cytoplasmic rigidity (vis- 

 cosity) either before or after cell formation. 



DEVELOPMENTAL CENTERS 



The Activation Center.— Seidel (19296) has used the term "Bild- 

 ungszentrum" to designate a region situated near the posterior pole of 

 the egg and necessary for development. This center is responsible for 



