EMBRYONIC RECONSTITUTIONS 515 



Pauli, 1927). It appears from recent work that loss of a certain amount 

 of ocplasm in consequence of puncture very soon after egg-laying does 

 not prevent normal development in Drosophila melanogaster, but with 

 similar loss at somewhat later stages abnormal imagines may result.*^ 



According to the conclusions reached by Seidel from ligaturing, local 

 cautery, and ultra-violet radiation of developmental stages of the libel- 

 lulid Platycnemis pennipes, the developmental pattern of this species dif- 

 fers from any pattern known in other animal groups. ^ The embryonic 

 primordium of this species becomes visible first as two lateral areas in 

 which nuclei are more numerous than elsewhere (Fig. 165, .4). These two 

 areas come together on the ventral surface; the cephalic lobes appear 

 anteriorly as still more densely nucleated areas and at the posterior end of 

 the primordium the level at which inversion of the embryo into the yolk 

 begins is also indicated by denser nucleation (Fig. 165, B). Seidel main- 

 tains that the region between the level at which the posterior end of the 

 embryonic primordium normally forms and the posterior end of the egg 

 is an initiative or formative center {Bildungszenirmn) and that a substance 

 essential to embryo formation passes anteriorly from it. Complete separa- 

 tion by ligature or kilhng of this region, during stages after cleavage 

 nuclei have reached the egg surface and before blastoderm formation, 

 prevents embryo formation but does not prevent further cell prohfera- 

 tion and formation of a cellular layer without definite pattern over the 

 whole surface anterior to the level of injury, a nonembryonic blastoderm 

 (Fig. 165, C). With complete separation by ligature at more anterior 

 levels, embryo formation occurs only posterior to the ligature and a uni- 

 form anaxiate cell layer anteriorly (Fig. 165, D, £); but with incomplete 

 hgature that does not destroy protoplasmic continuity, more or less de- 

 velopment may occur both anterior and posterior to the ligature, as in 

 Figure 165, F, in which a large head region develops anterior to, and an 

 elongated embryo posterior to, the hgature. As development progresses, 

 separation of the posterior region has progressively less effect on develop- 

 ment anterior to it; in late blastoderm stages separation or killing of 

 posterior parts has no inhibiting effect anteriorly. Incomplete ligature 

 which does not destroy protoplasmic continuity does not prevent or in- 

 hibit development anteriorly even in early stages, because, as Seidel main- 

 tains, the substance from the Bildungszentrum can still pass. 



As far as embryonic development is concerned, the primary effect of the 



^Howland and G. P. Child, 1935; Rowland and Sonnenblick, 1936. 

 7 Seidel, 1926, 1928, 1929a, b, 1931, 1934, i935- 



