286 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



ment associated together to form a suprarenal complex of the type 

 seen in the higher Vertebrates. Incidentally the unsuitability of 

 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 Amphioxus the sclerotome (Fig. 144 A, 

 scl) arises as a pocket-like diverticulum 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 

 myotome (cf. Fig. 144 C, scl). 



It must not be supposed that the whole of the connective 

 tissue in the body is necessarily derived from the sclerotome. On the 

 contrary it would appear that other regions of the mesoderm also 

 give rise to mesenchyme cells. Thus the inner surface of the 

 splanchnic mesoderm of the gut-wall would appear to give rise to the 

 connective tissue of this region, and the whole of the splanchnocoelic 

 mesoderm of the postanal region apparently becomes resolved into 

 mesenchyme. 



On the whole perhaps the safest position to take up is that of 

 regarding the power of forming mesenchyme as a general property of 

 the mesoderm, and of regarding the sclerotome merely as expressing 

 a localized concentration of this power, rather than as being the 

 representative of some primitive pocket-like diverticulum of unknown 

 function. 



