4» CHAPTER Vn 



f 



THE ASCIDIANS AND AMPHIOXUS 



THE ASCIDIANS Stand midway between the invertebrate and vertebrate 

 kingdoms. In the early development of many of them, a tadpole-like 

 larva is formed, which is furnished with an axial notochord with an 

 accompanying dorsal nerve-cord. These formations disappear or become 

 greatly modified in the fmal metamorphosed adult, but in so far as the 

 animal possesses them at all, it is to that extent entitled to be reckoned as 

 a member of the chordates. It is more doubtful whether the ascidians 

 should be regarded as exceedingly primitive members of the group in 

 which the larva foreshadows the later evolutionary history, or as a special- 

 ised group of degenerate forms in which the chordate larva is the only 

 remaining sign of a more glorious evolutionary past. Perhaps both views 

 have something of the truth; the ascidians may represent a chordate stock 

 which began to degenerate after only a short history of progressive evolu- 

 tion. In favour of this interpretation is the fact that in the very early 

 development of the egg, which leads to the formation of the chordate 

 larvae, they show many of the features which one would expect in a 

 primitive, as opposed to a highly evolved, member of the Chordata. The 

 eggs, in fact, bear a very striking resemblance to those o£ Amphioxus, 

 which is undoubtedly a very primitive chordate. It will, indeed, be 

 convenient to treat the early development of ascidians and Amphioxus 

 together. The simplicity and clarity of their developmental processes, 

 together with the primitive position of Amphioxus in the evolutionary 

 scheme, has for long rendered these animals classical embryological 

 material. 



The first full study of the Amphioxus egg was made by Hatschek in 

 1881. Unfortunately many of the authors who followed him (Wilson, 

 MacBride, Cerfontaine) misinterpreted the orientation of the early 

 embryo, thinking that the anterior was the posterior and vice versa; and 

 some textbooks still follow their erroneous accounts. The true state of 

 affairs was made out by the American embryologist Conklin (1932). In 

 the earher years of the century, the same author had laid the foundations 

 of our knowledge of ascidian experimental embryology (Conklin 1905). 



In both Amphioxus and the ascidians, the egg is fairly small (about o* i 

 to 0*2 mm. in diameter) and contains only a moderate amount of yolk. 

 Before fertihsation, the egg nucleus is in the form of a large germinal 



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