DEFECTIVE PLUTEUS LARV/E FROM ISOLATED 



BLASTOMERES OF ARE A CIA AXD 



ECU IN A RA CHNI t \S. 



HAROLD H. PLOUGH, 



DEPARTMENT OF BIOLOGY, AMHERST COLLEGE AXD THE MARINE BIOLOGICAL 



LABORATORY. 



ORGANIZATION IN THE SEA URCHIN'S EGG. 



Ever since Driesch in 1891 announced that a single blastomere 

 of the two cell stage of the sea-urchin's egg separated by shaking 

 would develop into a whole larva of half size, this method has 

 been generally used by embryologists for determining the type of 

 organization present in s^arious eggs at the time of the first or 

 later cleavages. Driesch's conclusions led to much controversy, 

 but most investigators have come to a fairly definite agreement 

 on the major facts to be learned from this field of investigation, 

 and have turned to other problems. The recent important work 

 of Yon Ubisch (1925) has once more drawn attention to the or- 

 ganization of the sea-urchin egg previous to cleavage, and has 

 made it. necessary to revise certain accepted ideas, The data 

 which I shall present appear to be of significance in this connec- 

 tion, and even though at this late date it would seem impossible 

 to add anything to the facts regarding the development of iso- 

 lated blastomeres of sea-urchin eggs, this will be my excuse for 

 briefly reviewing the evidence. 



In 1900 and in 1906 Driesch repeated and amplified his earlier 

 work and stated his conclusion that in the sea urchin egg there 

 was no evidence of differentiation at any cleavage stage, and that 

 the egg was an harmonic equi-potential system, each cell of which 

 might give rise to any part of the whole larva. In 1901 Boveri 

 studied the normal development and later isolated blastomeres, 

 and concluded that there was an entobla-t /one below the 

 equator of the egg and at right angles to its pnlar axis (colored red 

 in Paracentrotus], without some portion of which an isolated 

 blastomere would not develop an archenteron. He believed that 

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