Vol. VIII. March, lyoj. No. 4 



BIOLOGICAL BULLETIN 



ORGAN-FORMING SUBSTANCES IN THE EGGS OF 



ASCIDIANS. 



EDWIN G COXKLIX. 



WITH 24 PHOTOMICROGRAPHS OF LIVING EGGS OF CYNTHIA 

 (STYELA) PARTITA STIMPSON. 



BY KATHARINE FOOT AND E. C. STROBELL. 



That the egg of any animal is composed of " simple, undiffer- 

 entiated protoplasm ' is an article of traditional belief with a 

 large number of zoologists, and that the cleavage of the egg is 

 " a mere sundering of homogenous materials capable of any 

 fate" is a doctrine which has been given great prominence in 

 recent years. In favor of these commonly accepted views stands 

 a considerable body of experimental work on the development of 

 the ovum ; fragments of eggs or isolated blastomeres in many 

 cases are said to give rise to entire larvae, thus proving, as is 

 usually claimed, that the parts of the egg or embryo are still 

 undifferentiated at the time of the experiment. 



But not all experiments on development have confirmed these 

 conclusions ; some of the first and most careful researches of 

 this sort led to directly opposite results. In the development of 

 the frog's egg Roux found (1883, 85, 87, 89, 92, 93, 94, etc.) 

 that the median plane of the embryo is determined in the egg im- 

 mediately after fertilization and that "the development is, from the 

 second cleavage on, a mosaic work of at least four vertical in- 

 dependently developing pieces." In 1887 Chabry showed that 

 the early cleavage cells of the ascidian egg are specified for par- 

 ticular ends and that they develop, if they develop at all, into 

 parts which they,^Cvould produce under normal conditions. 

 These results were, however, denied on the ground of other ex- 



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