THE DEVELOPMENT OF LEPIDOSIREN PAHADOXA. 439 



This gradually increases in size (cf . Text-fig. 3), and about 

 Stage 32 communication with the exterior is established, 

 giving rise to the anterior nares. 



me. 



Text-fig. 3. — Transverse seclion near tlie anterior end of the head of a 

 Lepidosiren of St:ige 31. b.c. Rudiment of buccal cavity (still solid). 

 c.h. Cerebral hemispheres, me. Mesencephalon, olf. Cavity of olfac- 

 tory organ, not yet open to exterior. 



The posterior nares arise later about Stage 34, 



The Eye. 



The rudiments of the eyes begin to develop between Stages 

 20 and 21 as two solid "outgrowths" from the sides of the 

 brain rudiment (fig. 12, A). 



By Stage 23 a cavity has appeared in each optic process — 

 the fore-brain itself remaining still solid (fig. 12, B). The 

 cavity rapidly enlarges and becomes continuous with tlie 

 cavity of the fore-brain ; and at the same time, by differential 

 growth, the attachment of the optic stalk Ix^comes carried 



