66 THE AMPHIOXUS. 



Thus the closing of the gastrula along the central 

 line cannot be here quite so exactly observed as is 

 possible in other cases (e.g. Mollusca and Annelida). 

 A conclusion may however be drawn as to it by 

 means of exact consideration of the alterations in form. 



During the diminution of the gastrula-mouth in 

 the stage of Fig. 31 it may be noticed that the epi- 

 blast becomes ciliated. This is at first exceedingly 

 slight, and in the beginning only to be observed with 

 difficulty. By this the embryo gradually becomes 

 subject to a slow rotation. 



Kowalevsky stated that the embryo was first 

 covered with thick cilia, which he in some cases ob- 

 served to appear somewhat earlier than has here been 

 stated. In much later stages every epiblast cell bears 

 only a single flagellum. So far as my observation goes 

 every cell bears from the very beginning only one 

 single flagellum, very delicate, and later on continually 

 becoming longer. Of just the same condition are the 

 cells of the archenteron of the larva, which only 

 assume a ciliated appearance in a much later stage. 

 In this way no ciliated cells are to be seen in the 

 Amphioxus nor in the grown animal even during the 

 development, but only flagellate cells. 



