286 EMBRYOLOGY OF THE LOAVKI! VERTEBRATES CH. 



11 KM it associated together to form a suprarenal, complex of the type 

 seen in the higher Vertebrates. Incidentally the immutability o 

 the terms medullary and cortical is accentuated, for here when one 

 of the elements comes to surround the other it is the chromophile 

 which does so precisely the opposite to what happens in the 

 Mammalia. 



THE SCLEROTOME. In 'Ampliioxus the sclerotome (Fig. 144 A, 

 scl) arises as a pocket-like diverticuluni of the splanchnic mesoderm 

 just ventral to the myotome. It grows inwards and dorsal wards, 

 pushing its way between the notochord and spinal cord on the one 

 hand and the myotome on the other, until it reaches the mid-dorsal 

 line where it meets its fellow of the opposite side. The epithelial 

 walls of the sclerotome finally break up into mesenchyme amoeboid 

 connective tissue cells. The cells derived in this way from the 

 outer wall of the sclerotome apply themselves to the mesial face of 

 the myotome, penetrating in between its muscle cells and forming 

 septa of connective tissue between adjacent myotomes, while those 

 derived from the inner wall go to form packing tissue in the inter- 

 stices round spinal cord and notochord. Over the spinal cord this 

 packing tissue forms a tough protective roof. During this resolution 

 of the sclerotomes into mesenchyme all trace of the original 

 segmental character of the sclerotomes disappears. 



It is customary although the present writer regards it as 

 questionable whether this is wholly justified to regard the mode 

 of origin of the sclerotome seen in the developing Amphioxus as 

 representing the primitive mode of development. Upon this assump- 

 tion we may describe what takes place in the typical Vertebrates as 

 follows. The breaking up of the sclerotome into mesenchyme tends 

 to take place at earlier and earlier periods of development the 

 diverticulum stage becoming more and more transient and eventually 

 disappearing completely so that sclerotome formation comes to be 

 represented merely by a very active proliferation of mesenchyme 

 cells from the splanchnic surface of the mesoderm ventral to the 

 in yo tome (cf. Fig. 144 C, scl). 



It must not be supposed that the whole of the connective 

 tissue in tin; body is necessarily derived from the sclerotome. On t he 

 contrary it would appear that other regions of the mesodei in also 

 give rise to .n -srm -hyine cells. Thus the inner surface of the 

 spl;mehnie UK'S- (derm of the gut-wall would appear to give rise to the 

 eonneeiive tissue of this region, and the whole ni' the Bplanchnocoelic 

 :eriii of the poManal region apparently becomes resolved into 

 mesenchyme. 



On tin; \\hole |H-rh;i]is tin- safest position to take up is that of 



ling th- )OWT ot funning mesenohy me as a general property of 



tin- ni'-odei in, and of regarding the selerotomr merely as expressing 



;i |or;ili/ed rn|j< Ml I 1,1 1 lo| | of this |>o\\er. IMlhel' 1 ha II as heillg the 



BOf some primitive pocket like di\ ertieulum of unknown 

 funei ion. 



