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PRINCIPLES OF EMBRYOLOGY 



which diffusion could take place, the formation centre can still be effective 

 and the anterior part develop. This only occurs, however, if the con- 

 striction is made after the formation centre has become populated with 

 the dispersing cleavage nuclei. If, at an earlier stage, one makes a partial 

 constriction which is loose enough to permit diffusion of chemical sub- 

 stances but too tight to allow nuclei to pass, the formation centre can 

 never become nucleated, and it appears to be quite ineffective, since no 

 embryo is developed. One must conclude that the formation centre con- 

 sists of some speciahsed cytoplasm at the posterior end, but that it re- 

 quires to be activated by the passage into it of a cleavage nucleus (together 

 with the accompanying cytoplasmic halo); after being activated, the 

 formation centre gives out a chemical substance which diffuses forwards 

 and enables the main part of the egg to develop (Fig. 8.4). 



Figure 8.4 



The operation of the Formation Centre. If a very small part of the posterior 

 of the egg is constricted off at an early stage {a), an embryo can develop {b) ; 

 but if the constriction hes a Uttle further forward (c) no embryo forms (d). 

 After the formation of the blastoderm, however, an embryo is formed even 

 if the constriction lies well forward [e), since the Formation Centre has by 

 that time completed its action. (After Seidel 1929.) 



Seidel was able to alter the regular process of dispersal of the cleavage 

 nuclei, either by lolling one with localised ultra-violet irradiation or by 

 partially ligaturing the egg for some time so that the dispersal had to 

 occur in an abnormal space. He could in this way cause the formation 

 centre to be invaded by a nucleus other than that which would normally 

 have reached there; and he showed that any nucleus — or at least any of 



