38 OUTLINES OF CHORDATE DEVELOPMENT 



the two remaining connected by a very small tube. Not 

 until after metamorphosis does this opening become entirely 

 closed and the neuropore then remains as a ciliated funnel 

 known as the olfactory pit, which retains an innervation from 

 the elevated region of the brain and becomes sensory in char- 

 acter, the assumption being that it functions as an olfactory 



vd 



FIG. 13. Median sagittal sections through the brain of Amphioxus. After 

 Boeke. A. Of a larva with seven primary gill slits. B. Diagram of a section 

 through the brain of a larva of 2.25 mm., with five pairs of primary gill slits, 

 X 933. C. Same of a young specimen of 10 mm. X 233; c, Central canal of 

 cord; i, inf undibulum ; n, neuropore; p, cerebral pigment; op, olfactory pit; 

 v, cerebral vesicle; vd, postero-dorsal extension of cerebral vesicle. 



organ. This antero-dorsal outgrowth of the brain may cor- 

 respond with the lobus olfactorius impar which marks the 

 morphologically anterior end of the brain of the Craniates. 

 Just in front of and below this is the large cranial pigment spot 

 (Fig. 13) in the general region from which the olfactory lobes 

 grow out in higher forms. Postero-dorsally the brain wall is 



