198 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



splanchnocoelic portion of the mesoderm in the form of a series of 

 segmentally arranged sacs — the myotomes — and the wall of these 

 gives rise to almost the whole of the muscular system. The myotomes 

 are at first, from their mode of origin, restricted to the dorsal side of 

 the body, but as development goes on active growth of their ventral 

 portions takes place and they extend downwards, overlapping and 

 covering in the splanchnocoelic mesoderm right down to the mid- 

 ventral line. In this way a muscular body-wall is provided for the 

 ventral region of the body in which the original muscle-producing 

 capacity of the somatic mesoderm had disappeared. 



The evolutionary origin of this curious secondary muscularization 

 of the ventral body-wall of the Vertebrate is unexplained but the 

 suggestion may be hazarded that it was associated with the loss of 

 segmentation of the ventral splanchnocoelic mesoderm, the primitive 

 mode of movement of the Vertebrate — by waves of lateral flexure — 

 being only able to utilize longitudinal muscles divided into segments. 

 We may take it that the splanchnocoelic muscular layer, as it lost 

 its segmentation, would become less efficient for purposes of move- 

 ment, and that, correlated with this, its territory would then tend 

 to be encroached on by the still segmented, and therefore more 

 efficient, dorsal portion of the muscular layer until eventually it 

 came to be replaced completely by it. 



As a result of the developmental processes which have just been 

 indicated the mesoderm of Amphioxus, which for a time consisted of 

 a metameric series of paired sacs, is now represented by (1) the 

 segmentally arranged myotomes and (2) the unsegmented splanchno- 

 coelic lining. To these a third element becomes added in the form 

 of a pocket-like outgrowth from the myotome wall close to its lower 

 end (Fig. 144, A, scl, p. 285). This grows first towards the mesial 

 plane and then dorsally, insinuating itself into the space between 

 myotome on the one hand and notochord and spinal cord on the 

 other, until it occupies practically the whole of that space right up to 

 the mid-dorsal line. This pocket-like diverticulum is the sclerotome 

 (p. 286). 



In the typical Vertebrate a fourth derivative of the mesoderm 

 segment is of importance : it takes the form of a connexion which 

 persists for some time between the myotome and the splanchno- 

 coelic mesoderm as a narrow stalk or isthmus. This — the proto- 

 vertebral stalk or nephrotome (Riickert, 1888) with its cavity 

 the nephrocoele — is of great importance from its relation to the 

 nephridial organs but its existence has not up to the present been 

 demonstrated in Amphioxus. 



We will now proceed to trace out the subsequent late of these 

 various derivatives of the primitive mesoderm segments. 



Coelomic Cavities. — The only portions of the coelomic cavities 

 which remain patent are the nephrocoeles (which will be dealt with 

 later on) and the splanchnocoele or peritoneal cavity. 



It may be taken as probable that the body -cavity of the 



