626 



embryology: lancelet 



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Six of the first series of slits disappear, so that the number in 

 the two series is the same. While the formation of the second series 



of slits is taking place both series 

 shift downwards, the original series 

 passing over to the left side of the 

 body, while the second series 

 remains on the right. At the same 

 time the mouth also shifts to its 

 adult position in the middle line. 

 The slits at first open directly 

 upon the surface of the body, but 

 at an early stage, when there are 

 as yet only six clefts of the first 

 series, two longitudinal ridges 

 appear, one on each side of the 

 slits. In correspondence with the 

 position of the slits these ridges 

 lie in front on the right side of 

 the body, but behind curve down 

 to the ventral side, where the 

 new^ slits are forming. These 

 ridges are the metapleural folds. 

 From the inner face of each a 

 secondary ridge grows inwards to 

 meet its fellow and enclose a space 

 below the body. This is the rudi- 

 ment of the atrium. As the ridges 

 do not meet behind, there is left 

 an opening which becomes the 

 atriopore. The closure of the atrium 

 takes place from behind forwards 

 as the folds shift downwards with 

 the clefts, from the right side of 

 the body to their permanent posi- 

 tion (Fig. 485) . The atrium is at first 

 small, but enlarges so as to enclose the sides of the pharynx. The 

 endostyle appears at the beginning of the larval period as a band 

 of columnar ciliated cells on the right side of the anterior end of 

 the pharynx above the first gill-cleft. It becomes folded as a V 

 with the apex directed [backward. When the two rows of clefts 

 are established, the apex of this V grows back between them, the 



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