SIX FIGURES
Studies of marine ova have shown that the eggs of different species can be fertilized before, during, or after maturation. Four classes have been distinguished (Mead, 1898) : (1) those eggs fertilizable in the germinal vesicle stage; (2) those fertilizable at the metaphase of the first maturation division ;
(3) those fertilizable during the s9cond maturation division ; (4) those fertilizable only after completion of both maturation divisions.
Fertilizability may be defined as the capacity of the egg for effective fertilization, i.e., fertilization followed by development. Eggs in the fertilizable condition respond also to parthenogenetic agents. 0. and R. Hertwig (188'7) noted a complete failure of the fertilization reaction in primary oocytes of the sea urchin (class 4), even in those into which the spermatozoa had penetrated. Lillie ( '19) records similar observations upon unripe oocytes of Chaetopterus (class 2). The work of Delage (1899 a, 1899 b, '01) on the eggs of sea urchins, starfish (class 2 ) , the annelid Lanice, and the mollusc Dentalium (class 2) has shown that fragments taken from eggs with intact germinal vesicle ' Aided, in part, by a grant from the Smith Research Fund of the University of Xorth Carolina. I am indebted to Helen M. Costello for the draivings which illustrate this paper, and for assistance in the experiments. The experiments mere performed at the Marine Biological Laboratory during the summers of 1938 and 1939, and have been reported in abstract (Costello, '38 a, b, c).