44 AMPHIOXUS. 



The ova, which measure 0*105 mm. in diameter, appear to 

 escape from the atrial cavity, as a rule, through the atrial pore ; 

 but in some cases they have been seen to pass through the gill- 

 slits into the pharynx, and to make their exit through the 

 mouth. 



In the male, the testes are similar in form and position to 

 the ovaries of the female ; and the spermatozoa when ripe are 

 discharged, like the ova, into the atrial cavity, from which they 

 escape by the atrial pore. 



2. Morphological Importance of Amphioxus. 



It will be seen from the preceding account of its anatomy 

 that Amphioxus, while clearly and undoubtedly a Vertebrate, yet 

 differs from all ordinary Vertebrates, whether fish, amphibians, 

 reptiles, birds, or mammals, in a number of points which are of 

 great importance and affect almost every part of its body. 



A closer examination shows that these points of difference 

 between Amphioxus and the higher Vertebrates may be grouped 

 under two chief heads. 



(1) The atrial cavity, the large number of the gill-slits, the 

 regular alternation of gill-arches of two kinds, the azygos 

 character of the sense organs, the extension of the notochord to 

 the extreme anterior end of the animal, and the curious lateral 

 asymmetry shown by the myotomes, nerves, and other organs, 

 are examples of a group of characters in which Amphioxus 

 differs from the higher Vertebrates, not only in their adult con- 

 dition, but at all stages of their existence. 



(2) There is another and even more striking series of cha- 

 racters, in which Amphioxus differs from the adult forms of the 

 higher Vertebrates, but resembles these in their early develop- 

 mental stages. Thus in all the higher Vertebrates there is a 

 stage in development when the notochord is the only skeletal 

 structure present, neither cartilage nor bone having yet appeared ; 

 a stage in which the limbs are absent ; and a stage in which 

 the muscles of the body have the simple and definite segmental 

 arrangement seen in Amphioxus throughout life. In all higher 

 Vertebrates the heart is at first straight, like the cardiac aorta 

 of Amphioxus ; the liver arises as one or more outgrowths of 

 the intestine ; and the dorsal and ventral roots of the spinal 

 nerves are at first independent of each other. In these and in 



