38 ANATOMY OF AMPHIOXUS. 



The middle layer of the cutis below the epidermis (cf. 

 Fig. 1 8) is composed mainly of this tissue with radial 

 fibres superadded. In the metapleural folds it attains a 

 greater development than in the rest of the integument. 

 (Cf. Fig. 2.) It also constitutes the middle layer of the 

 sheath of the notochord, but the fibres in this case run 

 concentrically, and not radially.* The outermost layer 

 of the cutis (Fig. 18) and the innermost layer of the 

 sheath of the notochord are composed of a peculiar and 

 very highly refringent and homogeneous tissue of the 

 same order as that which forms the skeletal rods of the 

 pharynx. 6 The layer of connective tissue which separates 

 the myotomes from the body-cavity, and which springs 

 out from the base of the notochordal sheath (Fig. 2), occu- 

 pies the same position as the ribs of the higher Vertebrates. 



NOTES. 



i. (p. 15.) Metapleural Folds. --\\\ the development of the 

 paired fins of Selachians it was discovered, in 1876, by BALFOUR, 

 that at a certain stage there appears along each side of the body 

 "a thickened line of epiblast (i.e. ectoderm), which from the first 

 exhibits two special developments." " These two special thick- 

 enings are the rudiments of the paired fins, which thus arise as 

 special developments of a continuous ridge on each side, precisely 

 like the ridges of epiblast which form the rudiments of the 

 unpaired fins." After giving more details, Balfour says, "The 

 facts can only bear one interpretation, viz. that the limbs arc the 

 remnants of contimums lateral fins."' 1 



Shortly afterwards (1877), but quite independently, JAMES K. 

 THACHER was led by a comparative study of the adult skeleton of 



* In that portion of the sheath of the notochord which lies above the dor- 

 sal groove of the pharynx thers is a special tract of connective-tissue fibres 

 which run longitudinally. A similar tract can sometimes be observed in the 

 dorsal portion of the sheath below the nerve-cord. (Schneider, Lankester, 

 Spengel.) 



