202 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



For example in Birds 1 the mesodermal coating of the lung upon its 

 ventral side becomes continuous (1) with that lining the body-wall 

 laterally so as to enclose the portion of splanchnocoele dorsal to the 

 lung as a pleural cavity, and (2) with that covering the surface 

 of the liver, forming a ventral pulmonary ligament which serves 

 to wall in a pulmo-hepatic recess lying between it and the mesen- 

 tery. A third connexion, the, origin of which is associated with the 

 development of the abdominal air-sacs, forms the thin post-hepatic 

 septum which stretches from the ventral surface of the lungs 

 obliquely downwards and backwards to the ventral body-wall. 



Amongst Reptiles somewhat similar arrangements exist, differing 

 in detail in the different groups. 



The Myotomes. — The developmental changes by which, in a 

 gnathostomatous Vertebrate, the myotomes become converted into 

 masses of muscle-fibres are excellently shown by Lepidosiren in 

 which the cellular elements are particularly large and distinct. In 

 this animal the myotome is at first solid, but later on develops a 

 small cavity or myocoele by the breaking down of its central cells. 

 This myocoele soon becomes obliterated by its inner and outer walls 

 coming together. The cells of the inner wall assume a more regular 

 shape, taking the form of large parallelepipedal cells (Fig. 110, A, 

 mb'), flattened dorsiventrally and stretching in an anteroposterior 

 direction throughout the whole length of the myotome. The 

 nuclei of these large cells — myoblasts or myoepithelial cells — 

 divide, mitotically, so that they assume a syncytial character. 

 Their protoplasm develops a longitudinally fibrillated appearance 

 and presently distinct cross-striped contractile fibrils (mf) make 

 their appearance in the protoplasm — each fibril running through 

 the whole length of the myoblast or in other words from end to end 

 of the myotome. The contractile fibrils, which as seen in a trans- 

 verse section are arranged in a 3 -shaped pattern (Fig. 110, A, mf), 

 become more and more numerous and soon fill up the inner two- 

 thirds of the myoblast almost entirely, there remaining only a 

 relatively small amount of perifibrillar protoplasm between them 

 (Figs. 110, B, and 111). 



The outer end of the myoblast does not for some time develop 

 any contractile fibrils but there appear in its protoplasm large 

 vacuoles (vac) which form a broad clear band in horizontal sections — 

 of much use as a landmark to indicate the outer limit of the inner 

 wall of the myotome. The cells of the outer wall of the myotome 

 take the form of elongated cylinders stretching throughout the 

 length of the myotome and in their protoplasm longitudinal 

 fibrils make their appearance as in the case of the inner wall 

 myoblasts (Figs. 110, O; 111, B). The longitudinal fibrils become 

 fused at their ends with connective -tissue septa formed by mesen- 

 chyme cells which wander in between consecutive myotomes. 



1 For a well-illustrated account of the complicated arrangements in detail see 

 Poole (1909). 



