EGGS OF ASTERINA (PATIRIA) MINIATA. 



Ill 



age number of parthenogenetic eggs is about two per cent, of 

 maturated eggs. 



5. Eggs that undergo parthenogenetic cleavage never form 

 " fertilization " membranes, the closely fitting vitelline membrane 

 being the only envelope that surrounds the blastomeres. 



6. Cleavage in parthenogenetic eggs never begins earlier than 

 six and one half hours after the eggs are placed in sea water and 

 the average time for the beginning of cleavage is about seven and 

 a quarter hours. Cleavage begins in fertilized eggs sometimes 

 as early as three and one half hours, and the average time of be- 

 ginning is about four hours. Subtracting two hours for matura- 

 tion to complete itself, we have cleavage beginning five hours 

 after maturation in parthenogenetic eggs and two hours after 

 maturation in fertilized eggs. There is, therefore, a retardation 

 in development in the case of parthenogenetic eggs of three hours, 

 and at a very critical period. 



2 4 6 



FIGS. 1-6. Cleavage stages in spontaneously parthenogenetic eggs of As- 

 terina. i. An incompletely segmented two-cell stage in which one blastomere 

 is in advance of the other. 2. A normal two-cell stage. 3 and 4. Irregular 

 cleavage stages. 5. A typical normal four-cell stage. 6. A rather common 

 type of abnormal cleavage in which one blastomere is undergoing cytolysis 

 and the other is remaining normal. 



7. Cleavage and subsequent development in parthenogenetic 

 eggs take place much more slowly than in fertilized eggs. Even 

 the most nearly normal parthenogenetic eggs take nearly twice 

 as long to reach a given stage as do fertilized eggs. Develop- 

 ment is, therefore, greatly retarded and we would naturally ex- 



