EXPERIMENTAL EMBRYOLOGY 153 



parison with normals shows that none of these delays causes any effect 

 on later development other than delaying the onset. If, however, the 

 cleavage nuclei are prevented from entering this region by a constriction 

 of the egg, no development ensues, although the anterior parts continue 

 to live, undergo "yolk cleavage," and eventually form an extraembryonic 

 blastoderm. Partial constrictions show clearly that it is the nuclei that 

 must reach this area, not some chemical that can diffuse through the yolk 

 system. The reaction is not Hmited to the surface of the egg (which 

 lacks a distinct cortical layer), since peripheral parts in the region of the 

 center may be killed by ultraviolet Hght without preventing normal 

 development. Finally, the removal of constrictions and constricting 

 after the nuclei enter this region both show by the production of embryos 

 that the injury to the egg does not block development. Therefore we 

 must conclude (1) that the egg contains two quantitatively or qualita- 

 tively different regions (compare animal and vegetal poles of other eggs) 

 and (2) that neither the cleavage nuclei and their cytoplasm nor the 

 activation center is capable of producing a normal or even partial embryo 

 alone but that they react together to furnish a product that allows nor- 

 mal development to proceed along axial and symmetrical lines that have 

 already been laid down. 



Seidel further shows that after the nuclei have entered this region, 

 more and more can be constricted off from the posterior end of the egg 

 without blocking development. The action of the center spreads 

 slowly over a considerable area, so that well before the assembling of 

 cells to form the germ band its effect has been felt over the entire pre- 

 sumptive germ band (see curved line in Fig. 59) . The interaction product 

 must diffuse forward through the yolk system, since a partial constriction 

 after the nuclei reach the activation center does not prevent an embryo 

 from being formed anterior to the constriction. In some eggs there is 

 a visible change in the structure of the yolk proceeding apace with the 

 diffusion of factors from the posterior pole anteriorly. Furthermore, 

 this product activates the differentiation center which will be dis- 

 cussed below. 



In the ant Camponotus, Reith (1931, 1932) shows by constriction 

 and cauterization that an activation center is present near the posterior 

 pole, but he has not dehmited it closely. In this species it differs from 

 Platycnemis in that the cleavage nuclei are not responsible or even 

 necessary for its functioning. Its activation must be induced by some 

 product concerned in fertihzation, presumably produced not later than 

 the time of fusion of the egg and sperm nucleus, since the visible effect 

 of the functioning of the activation center is seen in very early cleavage 

 stages. This product must diffuse through the yolk system just as the 

 product of the activation center does, since neither is impeded by partial 



