Organisms from Eggs 145 



Conklin has established the existence of a definite 

 structure in the unfertilized eggs of Ascidians, Amphi- 

 oxus, and many molluscs. In all cases the results of 

 the isolation of the first blastomeres seem to agree with 

 the demonstrable structure of the unfertilized egg. 



5. These examples may suffice to show that the 

 egg has from the beginning a simple structure, and 

 w^e will now point out by which means further differen- 

 tiation may come about. Sachs suggested that all 

 differentiation and the formation of every organ pre- 

 supposes the previous existence of specific substances 

 responsible for the formation. These substances which 

 are now called internal secretions or hormones develop 

 gradually during embryonic development. What ex- 

 ists first is a jelly-like block of protoplasmic material 

 with a varying degree of viscosity and with just enough 

 differentiation to indicate head and tail end, a right 

 and left, and a dorsal and ventral side of the future 

 embryo. 



Aside from such simple differences phenomena of 

 protoplasmic streaming contribute to the further differ- 

 entiation. Such streaming begins, according to Conk- 

 lin, J in the egg just before fertilization when the surface 

 layer of the egg protoplasm 



1 Conklin, E. G., Heredity and Environment in the Development oj 

 Man. Princeton University Press, 1915. The reader is referred to 

 this book for the literature and main facts on the structure of the egg; 

 it should also be stated that Conklin's book is one of the best introduc- 

 tions to modern biology in the English literature. 



