148 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



anteriorly and ventrally near the locus of polar-body formation, and sug- 

 gests that a motivating impulse is transmitted from the differentiation 

 center following the release of the qualitatively distinct "Richtungs- 

 plasma" into the periplasm. There are, then, two possible origins of 

 this centrifugal movement, both hypothetical: (a) a more or less general 

 effect from the cortical layer or (6) qualitative differences not neces- 

 sarily included in the developmental centers although sometimes cor- 

 related with the differentiation center. Either is substantiated but not 

 proved by Strasburger's observation that Hght cauterization of the egg 

 of CalUphora frequently results in a displacement anteriorly of the 

 nuclear sphere. If either of these suggestions is true, then the origin of 

 the plasma streaming is one of the first indications of the activation of 

 the general dynamic egg system. 



With respect to stimulation to blastoderm formation there are four 

 noteworthy points: (1) Seidel's and Reith's observations that delay in 

 {Platycnemis) or failure of (Camponotus) cell-wall formation occurs after 

 elimination of the activation center; (2) Eastham's observation of the 

 formation of cell boundaries from the cytoplasm of the original cortical 

 layer; (3) Seller's observation of "cell" formation without nuclei but 

 with a thickening of the cortical layer; and (4) Strasburger's demonstra- 

 tion of the formation of cell walls at a definite time after entrance of the 

 nuclei into the cortical layer. The first suggests that cell formation is 

 aided or determined by the activation center; the second might be 

 interpreted as suggesting that there is a predetermination of the cortical 

 layer to form cells; the third must be interpreted as either a partial 

 activation causing both localized streaming and cell formation or a par- 

 tial activation causing streaming which in turn causes cell formation; 

 and the fourth suggests that cell formation is wholly a function of the 

 nuclei. The natural assumption is that all these factors are involved but 

 that they have different potencies in different insects. 



THE INDETERMINATE-DETERMINATE SERIES 



By differences in the time of appearance of the visible morphological 

 characteristics of development, especially the germ cells, Seidel (1924) 

 showed that it is possible to establish a series ranging from indeterminate 

 through incompletely determinate to determinate types. In indeter- 

 minate eggs the visible separation of regions of different prospective 

 significance occurs after blastoderm formation, and organ segregation 

 follows after differentiation of the germ layers and segmentation. In 

 determinate eggs this visible differentiation occurs before or during 

 blastoderm formation. 



Following this lead experiments were devised by Seidel and his asso- 

 ciates to determine if this series is also illustrative of the potencies of the 



