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ESSENTIALS OF ZOOLOGY 



nerve plate maintains its dorsal position and is carried back- 

 wards mid-dorsally, and after the formation of the neur- 

 enteric canal it is lengthened as a tube. All the mesoderm 

 is derived from the dorsal aspect of the endoderm and is 

 resolved into the entirely new formation, the notochord, and 

 coelomic segments. In the case of the paired somites the 

 early segmentation is noteworthy, and the differentiation into 

 myotomes and body cavity. It is also to be observed that the 

 germ cells appear in the myocoel. 



The larva bursts out of the egg membranes just after the 

 differentiation of the mesoderm commences. It is ciliated 





Intestine Notochord Anus Neurenteric canal Tail fin 

 FIG. 74. Amphioxus. Hinder end, illustrating the position of the anus 

 shortly after its formation and the shutting off of the neurenteric canal. 

 After Hatschek. 



all over, and the cilia maintain it in the pelagic state. In 

 the Mediterranean it takes only about eight hours to reach 

 this stage, and the larva is not yet able to feed. During the 

 next twenty-four hours it gains in length and, forming a 

 mouth, anus, and a gill slit, enters upon the larval period 

 proper, but as a larva which is characterised by a remarkable 

 asymmetry affecting the anterior ventral region of the body. 



The mouth is formed on the left side by the breaking down 

 of the two layers, and the opening gradually widens into a 

 large oval aperture. At the posterior end of the larva the 

 neurenteric canal becomes cut off from the enteron, and the 

 end of the intestine, thus freed, bends downwards to meet 

 and fuse with a slight diverticulum of the ectoderm and an 

 anus is formed. The larva is provided with a gill slit which 

 appears, like the mouth in front of it, by an absorption of the 

 two layers as a rounded opening situated on the left of the 



