EXPERIMENTAL EMBRYOLOGY 155 



of the egg (presumptive cervical region) at the site of polar-body forma- 

 tion, where the cortical layer is thickest, reticular cytoplasm is most 

 concentrated, cleavage nuclei become most numerous and first reach the 

 surface, vitellophags first appear, heterochronous mitoses first set in, 

 and cell partitions are first formed. A qualitative distinction of this 

 region during cleavage is indicated by differential staining with thionine 

 but not with haematoxylins. This region also has precedence in all 

 later differentiation (formation and closure of mesodermal furrows, seg- 

 mentation, appearance of appendages, etc.)- Spread of morphological 

 differentiation from a definite region is not so evident in the eggs of higher 

 Diptera. 



Experimental methods have demonstrated, as one might expect, 

 that this region of initial morphological differentiation is of fundamental 

 physiological significance, incorporating the so-called "differentiation 

 center." The results of constricting Platycnemis eggs (Seidel, 1934) show 

 that this center normally coincides with the starting point of morpho- 

 logical differentiation, though seemingly this morphological manifestation 

 can be experimentally separated from the primary physiological center. 

 The differentiation center extends from the second gnathal (maxillary) 

 segment to the second thoracic segment with its mid-point in the anterior 

 half of the presumptive prothorax (Figs. 59c, 62a). 



Seidel (1929&, 1931, 1934) further shows that no differentiation can 

 occur in an isolated region of the egg of Platycnemis unless the differen- 

 tiation center is present in whole or in part. The center can function 

 only after the product of the activation center has reached it by diffusion. 

 It does not affect blastoderm formation but does directly influence the 

 assembling of blastoderm nuclei and heterochronous divisions in germ- 

 band formation, although heterochronous divisions alone occur when 

 almost the entire center is tied off. Unlike the activation center, the 

 differentiation center cannot function normally with even a relatively 

 slight constriction of this region of the egg. A very loose ligature out- 

 side the boundaries of this center will allow the formation of the germ 

 band on both sides of the ligature; but if it is only slightly tighter, even 

 though the blastoderm and yolk remain unsevered, the germ band forms 

 on one side only. Since no continuity is destroyed in any part of the 

 egg by such a constriction, the center's action cannot depend solely upon 

 the spread of a substance but must involve an energy transfer, i.e., must 

 be a dynamic phenomenon. Cell aggregation to form the germ band is 

 not prevented by killing a complete girdle of blastoderm cells over the 

 entire region of the center (by ultraviolet irradiation). Initiation of 

 this process must therefore originate in the "yolk system," viz., the yolk 

 with its included cytoplasmic reticulum and vitellophags. Localized 

 contractions of the surface of the yolk system, produced by cautery or 



