No. 3-] THE GASTRULATION OF AMPHIOXUS. 595 



of Ciona intestinalis " is much more detailed than the work of 

 previous writers and we shall confine ourselves entirely to 

 Castle's account. 



In Ciona, at the time when the endodermal plate is bending 

 in to form the archenteron (Castle, Fig. 78), a few cells at the 

 dorsal lip of the blastopore also sink in and, in later stages, 

 form the dorsal wall of the archenteron (Fig. 98). From these 

 cells the notochord subsequently develops. They would seem, 

 therefore, to correspond to the cells of the dorsal wall of 

 Amphioxus. Outside of the semicircle of notochordal cells 

 (Fig. 62), at the dorsal lip of the early gastrula, lie the ecto- 

 dermal cells that subsequently form the nerve plate. As the 

 blastopore closes, these cells are carried backward with the 

 advance of the dorsal lip, but none of the cells turn in with 

 the notochordal cells. During the backward growth of the 

 ectodermal and notochordal cells, these cells increase in num- 

 ber. The nervous system of Ciona is derived from cells that 

 lie at first in front of the dorsal lip of the blastopore. The 

 nervous system of Amphioxus is also derived from cells in 

 front of the dorsal lip of the blastopore, but in Ciona the 

 blastopore closes from before backward, while in Amphioxus, 

 if our view be correct, the closure is not from before back- 

 ward, but equally from all points of the periphery. It is im- 

 portant to note that in Ciona, in which the advance of the 

 dorsal lip is definitely shown to exist, the ectodermal cells that 

 lie at the free edge of the dorsal lip do not turn in during the 

 period of closure. 



Castle derives the mesoderm of the tail from a number of 

 cells around the posterior half of the blastopore. These cells 

 are turned into the archenteric space during the period of gas- 

 trulation. Castle has followed the lineage of these cells and 

 has shown that they correspond in origin with the ectodermal 

 cells of the anterior region that form the nervous system. He 

 therefore considers them ectodermal in origin, and consequently 

 derives a large part of the mesoderm from the ectoderm. The 

 more anterior mesoderm of the trunk comes from cells lying 

 just within this semicircle of mes-ectodermal cells. This inner 

 circle is described as endodermal in origin. Hence, in Ciona, 



