130 DEVELOPMENT OF AMPHIOXUS. 



this region. (Cf. Figs. 65 G and 75.) I have shown that 

 this epithelial tract is the rudiment of the endostyle (vide 

 infra]. 



It is a curious fact that the first trace of pigment to 

 appear in the nerve-tube is not the eye-spot, but that at a 

 constant point in the region of the fifth somite a black 

 pigment-spot is deposited in a cell in the ventral wall of 

 the medullary tube. This is followed by another smaller 

 pigment granule slightly posterior to the first (Fig. 63 C). 

 The eye-spot appears at the end of the embryonic period. 



LARVAL DEVELOPMENT. 



Formation of Primary Gill-slits, etc. 



With the establishment of the definite relations oi the 

 head-cavities, the mouth, club-shaped gland, first gill-slit, 

 and anus, the embryo enters upon the larval phase of the 

 development. 



It is no longer, or only very rarely, to be taken from 

 the surface of the sea, but descends to a depth of several 

 fathoms. When kept in aquaria, the larvae can often be 

 observed to be suspended vertically, and apparently quite 

 motionless in the water. This suspension is, no doubt, 

 effected by the movement of the long cilia, or flagella, 

 with which the ectoderm is provided, each cell possessing 

 one flagellum. 6 



The principal changes which take place during the early 

 stages of this phase of the development are the addition of 

 new myotomes, the formation of new gill-slits, in meta- 

 meric order, in an unpaired series on the right side of the 

 larva, to the number of from twelve to fifteen, or even 

 sixteen (the more usual number being fourteen), and the 

 origin of the atrial cavity. 



