EMBRYOLOGY 



483 



eight in number. 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 down- 

 wards, 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 rudiment 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 



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