86 
MORGAN. 
PLATE VI. 
Longit. section, vertical. Opening of first gill-slit, etc. 
Longit. section, vertical. Opening of second gill-slit. Same series. 
Longit. section, vertical. Same series, nearer middle line. 
Longit. section, horizontal, of Stage VIII. 
Surface view from within of nerve-cord. 
Surface view from within of nerve-cord. Younger. 
Cross-section of collar to show nervous system, eéc. 
Cross-section of first gill-slit. (Horizontal series.) 
Cross-section of nerve-cord, notochord, e?c. 
Cross-section of gill-region. (Horizontal series.) 
Cross-section of neck. 
Cross-section of collar-pore. 
Youngest stage of development of gonad from body-wall. 
Older stage of development of gonad from body-wall. 
Oldest stage observed of development of gonad. 
