320 PROCEEDINGS OF THE ACADEMY OF [July, 



THE EMBRYOLOGY OF FULGTJR: A STUDY OF THE INFLUENCE OF YOLK 

 ON DEVELOPMENT. 



BY EDWIN G. COXKLIN, 



PROFESSOR OF ZOOLOGY, UNIVERSITY OF PENNSYLVANIA. 



I. Introduction. 



1. GENERAL. 



It is generally believed that the accumulation of yolk in the eggs 

 of certain animals has greatly modified their original manner of develop- 

 ment. For example, selachians, reptiles and birds are probably 

 derived from animals in which the eggs contained much less yolk 

 than at present in these classes, and in which the development was 

 different in many respects from that which now obtains among these 

 forms. The effect of the loss of yolk upon development has been 

 studied experimentally by Morgan (1S93), but the influence of an 

 increase in the amount of yolk, while the protoplasmic portion of the 

 egg remains the same, has not been studied experimentally, and may 

 perhaps lie beyond the test of experiment.^ Such experiments, how- 

 ever, nature has performed in several different phyla of animals, but 

 nowhere are natural conditions more favorable for a study of the 

 influence of yolk on development than among the mollusks. Brooks 

 (1S79) long ago called attention to this fact, though it has not been 

 reinvestigated in the light of the modern cellular study of develop- 

 ment. 



The comparison of the development of large eggs with that of small 

 ones, with especial reference to the organization of the egg and the 

 history of the cleavage cells, should be of considerable general interest. 

 Does a great accumulation of yolk change the localization of morpho- 

 genetic processes and substances in the egg? Does it alter the devel- 

 opmental history and destiny of the blastomeres? Is the yolk itself 

 localized in any definite germinal region of the egg? In what manner 

 does a great mass of yolk alter the gastrulation and later embryonic 

 development? These are questions upon which a detailed comparison 



' The fusion of different eggs to form giants, such as occurs abnormally in 

 certain cases, is not a case in point, since the relative amount of yolk and pro- 

 toplasm remains the same in the fused eggs as in the single ones. 



