446 THE EVOLUTION OF MAW. 



By the motions of these delicate vibratory hairs, the gastrula 

 of the Amphioxus, like that of many other animals of low 

 organization, after it has broken through the egg-coveringb 

 rotates and swims in the ocean (Fig. 156). 



In the course of further development the roundish Bell- 

 gastrula of the Amphioxus lengthens, and at the same time 

 it becomes rather flatter on one side parallel to the longi- 

 tudinal axis. The flattened side is afterwards the dorsal- 

 side ; the opposite ventral side remains roundly arched. In 

 the middle of the dorsal surface appears a shallow longi- 

 tudinal furrow or channel (Fig. 157), and on each side of 

 this channel the surface of the body rises in the shape of 

 two parallel ridges or longitudinal swellings. I need 

 hardly say, that this channel is the primitive groove, or 

 dorsal furrow, and that these swellings are the dorsal 

 swellings or spinal swellings which form the first rudiments 

 of the central nervous system, the medullary tube. These 

 two swellings grow higher and higher ; the groove becomes 

 deeper and deeper. The edges of the two parallel swellings 

 incline towards each other, and finally coalesce, and thus 

 the medullary tube is completed (Plate X. Fig. 11, m). The 

 formation of the medullary tube from the outer skin takes 

 place, therefore, on the naked dorsal surface of the independent 

 Amphioxus larva in exactly the same way as in the embryo 

 of Man and of other Vertebrates within the egg-envelopes. 

 In both cases, also, the nerve-tube finally separates entirely 

 from the horny plate. The fact is peculiar, that at that 

 end of the body which afterwards is to be the anterior or 

 mouth end of the Amphioxus, the medullary tube remains 

 open at first, and has an external opening (Fig. 11, ma). 



Even at the time when the first trace of the dorsal furrow 



