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THE ORGANIZATION CENTRE 73 



occurs quite soon after fertilization, when the 

 cleavage cells are still rather large; and if one 

 attempts to make grafts such a large proportion of 

 cells get injured in the operation that the experiment 

 is not a success. So Horstadius did his operations still 

 earlier, during the cleavage, when he could cut in 

 between the cells without injuring them. Most of the 

 experiments were made when the egg had divided 

 into sixteen cells. The cleavages are not quite equal, 

 and the sixteen cells are arranged in three circles, one 

 at the top consisting of eight moderate-sized cells, 

 then a circle of four large cells, and at the bottom a 

 circle of four small cells (Fig. 19). Horstadius cut the 

 embryo in half in various ways and then joined the 

 halves together again. He found that several, but not 

 all, combinations of halves could regulate themselves 

 so as to produce a normal larva. He also found that 

 wherever the small vegetative cells were grafted, they 

 started sinking in and turning into endoderm, and 

 moreover they carried the surrounding cells in with 

 them and induced them to become endoderm. They 

 therefore acted to some extent like an organizer. We 

 shall discuss in Chapter vi the way in which this 

 organization centre seems to differ from that of the 

 newt. 



Insects 



In the insects another sort of organization centre has 

 been found which is still more different from that 

 of the newt. It was discovered by Seidel, who worked 

 on dragon-flies' eggs. We have not said anything 



