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THE AMERICAN NATURALIST [Yol.XLVII 



the vesicle becomes elongated laterally so that its right 

 and left sides come to be separated a maximum distance 

 from each other. In such a depressed and weakened 

 vesicle unity of functioning ceases to exist and two new 

 centers of growth arise at points where the pressure is 

 less severe, viz., the opposite ends of the elongated vesicle. 

 We have seen that mesoderm forms at two lateral points 

 and that the embryonic buds of the ectodermal vesicle 

 follow suit. The rebudding of the primary buds must 

 be due in like manner to the establishment of two grow- 

 ing points in each primary bud. Such an explanation of 

 polyembryony involves the whole problem of the physiol- 

 ogy of budding, about which there is great diversity of 

 opinion. According to Professor Child's theories of 

 development and reproduction, any part of a system 

 which, through a lowering of the rate of metabolism of 



