544 CEPHALOCHORDA. 



canal, viz., the filling up of the gap in the entoderm-sac which arises 

 through the abstriction of the chorda and the primitive segments. 

 On this point, however, LWOFF (No. 17) does not agree with 

 HATSCHEK. 



C. Further Development up to the Time when the Mouth 

 and the First Gill-cleft Form. 



The embryo quits the egg-envelope very early, usually at about 

 the stage in which the rudiments of two pairs of primitive segments 

 can be recognised (Fig. 281). The young larva then swims about 

 freely by means of its cilia, its movements recalling those of many 

 invertebrate larvae. As the body moves forward it rotates round its 

 longitudinal axis, thus taking a spiral course. 



The transition from embryonic to larval life is very gradual in 

 Amphioxus. The larva is capable of feeding independently only 

 in comparatively late stages. Up to that time it is nourished by 

 particles of the food-yolk derived from the egg which, in the form 

 of spherical granules, fill all the tissues of the embryo, but do not 

 disintegrate simultaneously in the different layers of the body. They 

 disappear most quickly in the external epithelium, then in the meso- 

 derm, then in the medullary plate, and are retained longest in the 

 entoderm. 



In an aquarium, the newly hatched larva for the first few hours of 

 its free-swimming existence keeps rigidly to the surface of the water. 

 At later stages, however, the larva frequently rests at the bottom, 

 lying on one side. Thus, during metamorphosis, Amphioxus leads 

 a pelagic life. The young Amphioxus, when metamorphosis is com- 

 pleted, buries itself in the sand in an upright position (WmLEY), 

 like the adult, only the anterior end of the head projecting above the 

 surface. 



In the gastrula-stage, the embryo becomes ellipsoidal, the dorsal 

 side at first flattening or curving in (medullary groove, Fig. 280 (7, 

 281). Later, the embryo increases in length, the body at the same 

 time becoming laterally compressed (Fig. 286). As the anterior end 

 of the body lengthens out into a snout, and the posterior end becomes 

 marked by a caudal fin, it bears a general resemblance to a fish (Figs. 

 288, 290). 



As the body increases in size the ectodermal cells change, as a rule, 

 into flattened, ciliated cells, only remaining columnar at the anterior 

 and posterior ends of the body. The larval (primary) caudal fin (Figs. 



