198 



COMPARATIVE ANATOMY 



called, are at first solid, but become hollowed out by the two 

 metapleural lymph spaces. 



The mesoblastic, somites are shown in fig. 49 as triangular 

 sacs containing very small cavities, and lying right and left 

 of the notochord. Fig. 49, D, shows how these sacs have 

 extended dorsally and ventrally between the epiblast and the 

 gut, notochord, and nerve tube. At the same time their 

 cavities have enlarged greatly, so that we may distinguish an 



f-f 



Fig. 50 



A . Diagrammatic ventral view of a larval Amphioxus illustrating the 

 formation of the atrial cavity, af, atrial folds ; ap, atriopore ; gs, 

 gill-slits ; m, mouth ; //, pre-oral pit. B. View of the first eight 

 somites of a larval Amphioxus with three gill-slits and a rudiment 

 of a fourth ; from the right side. /-<?. mesoblastic somites ; /-///, the 

 first three gill-slits ; IV ', the rudimentary fourth gill-slit ; eg, club- 

 shaped gland ; e, eyespot ; g, glandular tract ; //, pre-oral pit. 

 (After Lankester and Willey.) 



outer layer of cells or somatopleur lying against the epiblast, 

 and an inner wall or splanchnopleur lying against the gut and 

 notochord. But the cells of that part of the inner layer which 

 lies against the notochord become differentiated at a very 

 early period in the manner shown in fig. 49, D. They become 

 elongated and extended in a longitudinal direction, so that 

 they nearly fill up the upper part of the ccelomic space. These 

 cellular growths are the first signs of the myotomes. At a 

 later stage, shown in fig. 49, E, the lower limbs of the pouches 

 have grown down to the mid-ventral line, and fused below 

 the gut, so that the latter is surrounded below by a coelomic 



