XVII 



PEOTOCHOEDATA 



599 



vertebrate phylum as compared with 

 fish that its tail-growth is insignificant. 



In older larvae a slight ectodermic 

 thickening along the Line of the posterior 

 extension of the first somite, on the 

 right side, is observable ; this thicken- 

 ing is the first rudiment of the atrial 

 fold, which overhangs the gill -slits. 

 A similar fold is developed on the left 

 side at a later period of development, 

 and during the metamorphosis into 

 the adult form these two folds unite 

 beneath the ventral surface of the 

 animal and enclose the atrial cavity. 



Hollow outgrowths from the inner 

 walls of the myotomes give rise to 

 tubes which insinuate themselves 

 between the myotomes on the one hand 

 and notochord and nerve cord on the 

 other (Hatschek, 1888). These are 

 the lower sclerotomes (scl 2 , Fig. 

 438) ; from their inner walls a fibrous 

 sheath is developed which enswathes 

 notochord and nerve cord. Their 

 outer walls furnish a sheath, or fascia, 

 for the muscular fibres of the myo- 

 tomes. 



From the uppermost angles of the 

 myotomes similar outgrowths arise 

 which we may term the upper sclero- 

 tomes (scl 1 , Fig. 438), and become com- 

 pletely cut off from the myotomes. Thus 

 they give rise to a series of pockets Lined 

 by coelomic cells which are termed the 

 fin-ray cavities. These cavities were 

 described by Hatschek (1888); their 

 origin from the myocoele was first seen 

 by Goldschmidt (1905). The cavities 

 are arranged in a single series in the 

 mid-dorsal line, four or five occupying 

 a space corresponding to the breadth 

 of a myotome. They appear to be 

 formed by outgrowths arising alter- 

 nately from the myotomes on the right 

 and left side. The ventral wall of 

 each becomes thickened and forms 

 the gelatinous peg known as the 

 fin-ray. 



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