GENESIS OF MAN. 41 



longs to a great subtype of the Vcrtcbrata, which Haeckel denomi- 

 nates the Acrania. This entire sub-type has now but a single 

 known living representative on the globe, the amphioxus 9 ; but 

 Haeckel believes that a period existed when the Acrania greatly 

 prevailed over the Craniota, or cranium-bearing vertebrates, and 

 peopled all the seas and waters. This Acrania or amphioxus form 

 constitutes the seventh stage of ontogenetic development. 



The next or eighth stage is the Lamprey or Monorrhina stage. 

 The nervous system and the vertebral column begin to differen- 

 tiate. The spinal marrow undergoes a slight enlargement at its 

 anterior extremity, which is the rudiment of the brain. The 

 vertebral column begins to develop out of the rudimentary 

 chorda dorsalis. This does not take place by a gradual, simulta- 

 neous formation of all the vertebrae along the line of the chorda, 

 but, singular as it may seem, by the formation of one vertebra 

 after another, beginning with the most anterior. This remarkable 

 process points unmistakably to the composite character of the 

 frame-work of every vertebrate body. Each vertebra of a verte- 

 brate, like each ring of an annelid, represents a distinct and once 

 independent unit of a compound organism. 



The present Cyclostomata or Monorrhmae 3.re believed by Haeckel 

 to be the sparse remains of a once great group of animals, which in 

 ancient times shared the possession of the globe with their gradually 

 increasing rivals, the Amphirrhinae, which had sprung from them 

 just as the embryo of every higher vertebrate passes from the con- 

 dition of the one into that of the other. The type of the former is 

 the still living Lamprey or } etromyson. As the names imply, the 

 Monorrhinae have but one orifice for mouth and nose, which is of 

 a circular shape, and is used as a sucker, while the Amphirrhinae 

 are provided with a pair of jaws and two nasal orifices. Excluding 

 the Amphioxus (Acrania) the entire vertebrate type ( Craniota) falls 

 under these two groups, the Amphirrhinae embracing all the higher 

 vertebrates, from the lower fishes upward. 



From the form of the first of these groups to that of the second 

 the embryo now passes, and enters upon its ninth stage of develop- 

 ment; it becomes a fish. But as natura non facit saltiun, this first 



9 A second acranial animal, discovered near Peale Island, Morton Bay, Australia, 

 has very recently been reported to the Royal Society by Prof. W. Peters, who has de- 

 scribed it under the name of Epigomethys cultellus. This discovery is of the highest 

 interesfto naturalists. 



