|-:\l'L.\N.\riiiN III' I'LA'I'K ILL! STHATIMi THE 

 l':Mr.KV(iLn(,V OK OSTJIKA CLoMiniATA. 



Figs. 1 iiud 2. I'll fertilized ova with investing vitelline membrane 

 and micropyle, a : magnified about OUO dia- 

 meters. 



,, '6. Milt or sperm cells. 



,, I. (J\ um with iittached s[)eriii cells, immediately 



preceding amalgamation and fertilization. 



.■>. Ovum from 1st to 2nd hour after fertilization. 



,, (i and 10. Consecutive metamorphoses observable within the 

 2nd and 3rd hours after fertilization. 



,, 11 and 13. Changes progressing within the 4tli to Hth hours 

 after fertilization, terminating in the ejection of 

 the polar cell, //, and the complete investment 

 of the larger nutritive cells, hypoblast, liy the 

 smaller formative cells or epiblast. 



,. 1 1 and I'i. L'hises of thi embryo, known as the " Castrula " 

 stage, observed lietween the 10th and 1-jth 

 hours of development. 



,. ITand ]w. Phases arrived at from within 2-4 to 4.8 hours, 

 and in which the shells, sJi, mouth, m, and vent 

 '■, of the perfect oyster have made their 

 appearance. 



,, r.).iiil 20. Lateral and dorsal aspects of an embryo, three to 

 four days old. and in which the shells have 

 grown to sucIj dimensions that they entirely 

 enclose the body when contracted ; magnified 

 about 200 diameters. 



,, 21. 1-^arliest observed attached condition of the oyster 

 endnwos or " spat," attained to within the 

 fourth or fifth days succeeding the primary 

 fertilization of the ovum ; magnified about -30 

 diameters. 



