136 DEVELOPMENT OF AMPHIOXUS. 



ceed. With their appearance, the larva enters upon that 

 phase of its development which has been called the later 

 larval period. It is the period of the metamorphosis of 

 the larva, during which the pronounced asymmetrical 

 arrangement of the parts is exchanged for the partial, but 

 not absolute, symmetry which we have noted in the adult. 

 The metamorphosis, therefore, consists largely in the sym- 

 metrisation of the larva. 



The simultaneous appearance of the six nodal thicken- 

 ings in the exact position, shown in Fig. 74, is very 

 constant. Shortly afterwards a minute perforation appears 

 in the centre of each thickening almost simultaneously, 

 except in the case of the first, which usually becomes 

 perforated rather later than the others. The originally 

 small circular openings of the secondary gill-clefts gradually 

 increase in size and become oval in shape, their long axes 

 being parallel to the long axis of the body, instead of at 

 right angles to it as in the case of the primary slits. 



Next, the upper borders of the secondary slits begin to 

 flatten, and later to show signs of curving downwards. 

 The changes in shape, which affect the secondary slits at 

 the stages now under consideration, may be expressed by 

 saying that they are at first shaped like a biconvex lens, 

 then like a plano-convex lens with the flat surface directed 

 upwards and the convex surface downwards, and finally 

 like a concavo-convex lens with the concavity directed 

 upwards (Fig. 77). 



During these changes, which do not take place in all the 

 secondary slits at the same time, the last one especially 

 retaining for a long time its primitive shape, the walls of 

 the successive slits become sharply rounded off and distinct 

 from one another, and a new perforation makes its appear- 

 ance in front, above, and between the second and third 



