The Development of Iscbnochiton. 575 



remaining products of this quartette and the macromeres constitute 

 the entoderm. 



Up to the formation of the mesoblast in the 72 cell stage a 

 perfect radial symmetry exists. At this time however certain divisions 

 in the posterior second quartette (first somatoblast) arise which to 

 some extent destroy it, but in the upper hemisphere, on the other 

 hand, radial symmetry persists until at least 150 cells are formed. 

 Bilateral symmetry appears therefore at a rather late stage in the 

 development, and after its appearance the transition from the radial 

 to the bilateral condition is slow; in fact traces of the former persist 

 until the close of the free swimming period. 



Gastrulation occurs almost entirely by invagination, which proceeds 

 most rapidly on the anterior side of the blastopore where the cells are 

 smaller. In the rapid movement of this process the mesoblast becomes 

 pushed backward, almost posterior to the archenteric wall, and with 

 the shifting invagination on the posterior side practically ceases, while 

 it continues on the anterior side with unabated vigor. Rapid cell 

 divisions now arise on the posterior side of the embryo in the ecto- 

 dermal cells whereby the distance between the posterior borders of 

 the blastopore and prototroch on the dorsal side becomes constantly 

 increased. In proportion as this increase takes place a corresponding 

 decrease of the surface between the anterior border of the blastopore 

 and prototroch occurs by means of the invagination process. Finally 

 the ectodermal tract between the blastopore and velum enters into 

 the formation of the stomodaeum, and the mouth consequently comes 

 to lie immediately behind the prototroch. 



During the first stages of the shifting of the embryonic axes, the 

 prototroch, consisting of thirty-four cells arranged in a double row 

 around the embryo, becomes ciliated. As in the Annelids, a gap at 

 first occurs on the dorsal side, but this is filled by two cells from 

 the anterior hemisphere. This organ continues functional until after 

 the free swimming stage is passed, when it becomes pushed out of 

 its position and is cast away. 



By a slight invagination a few cells in the centre of the anterior 

 hemisphere sink in a short distance and become attached to the 

 cerebral ganglia, which arise as two ectoblastic thickenings in the 

 head vesicle (Fig. 48). From the bottom of this depression two 

 compound flagella arise, the whole structure forming the apical sense 

 organ. 



On the ventral side just posterior to the mouth the foot arises 



