532 BERTRAM G. SMITH. 



The immature post-larval stages resemble in coloration the 

 young adults described in Part I. Aside from the loss of the 

 external gills, the most conspicuous changes as compared with 

 the larvae are. a slight progressive dorso-ventral flattening of 

 the body, and the gradual development of the folds of the skin 

 which are so prominent in adult and especially in very large 

 and presumably old specimens. A transverse slit-like spiracular 

 opening is bordered anteriorly by the small opercular flap and 

 posteriorly by a similar but still smaller fold of skin. The spiracle 

 leads to a small cavity containing a single persistent gill opening 

 bordered by two leaf -like plates; the other gill-lamella is greatly 

 reduced or has disappeared. 



D. Summary 



The vitelline membrane is shed about the time of the closure 

 of the neural folds. 



Shortly after the closure of the neural folds, the entire surface 

 of the embryo becomes ciliated. The beat of the cilia is in gen- 

 eral toward the posterior end of the body; currents of water are 

 produced which are subservient to respiration. After the appear- 

 ance of the external gills, ciliary currents are especially strong 

 in their immediate vicinity. Soon after the hatching period, 

 the cilia disappear except on the external gills ; here they persist 

 until at least six months later. 



Soon after the closure of the neural folds, the embryo falls on 

 one side, where it lies until, in a much later stage, it is able to 

 turn over through muscular activity. The embryo falls on the 

 right or the left side in about an equal number of cases. 



Rotation of the embryo, due to the beating of the ciHa, com- 

 mences before the embryo has fallen on its side but is more pro- 

 nounced afterward. In most cases rotation proceeds in a clock- 

 wise direction when an embryo is lying on its right side, in an 

 anti-clockwise direction when it is lying on its left side. Rotation 

 may be of service in preventing adhesion of the embryo to the 

 capsule. 



The vitelline veins develop as paired structures along the 

 sides of the yolk sac, and shift gradually toward the ventral 



