GENERAL ZOOLOGY 



Unfertilized egg \^\ .' * 

 Nucleus 



Fertilization 



Initiation of 

 division 



Two-cell stage ^^ .,; • 



Four-cell stage 



Mature umbo 

 stage 



Larva using 

 t: foot for 



feeling on 

 bottom 



Spat 2 or 3 hours 

 after attachment 



Adult shell 



5 or 6 hours 

 after attachment 



Early umbo 

 stage 



About 2 to 3 days 

 after attachment 



1.0 mm. 



Fig. 13.5. Developmental stages of the oyster, Crassostrea mrgimca. Note the proajressive 

 loss of bilateral symmetry just before and after attachment of the young oyster. The stage 

 marked "Beginning of shell formation" corresponds essentially to the trochophore stage; the 

 "Straight-line larva" is a veliger. (From C. L. Newcombe and R. W. Menzel, 1945, Contribu- 

 tion No. 22, Virginia Fisheries Laboratory.) 



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