THE ORGANS OF THE MIDDLE GERM-LAYER. 405 



During its development some small portions of the fundament of 

 the suprarenal cortex appear sometimes to detach themselves and to 

 remain in. the vicinity of the sexual organs, in whose migrations they 

 participate. Thus, indeed, are to be explained the accessory supra- 

 renal bodies observed by MARCHAND at the margin of the broad 

 ligament. 



SUMMARY. 



1. The following structures are to be interpreted as formative 

 products of the middle germ-layer : the epithelium of the body-cavity 

 (of the pericardium, of the thoracic and abdominal cavities, of the 

 cavity of the scrotum), the whole of the transversely striped, voluntary 

 musculature, the seminal cells and ova, the epithelium of the sexual 

 glands, of the kidneys and their outlets, and the cortical cords of the 

 suprarenal bodies. 



The Development of the Musculature. 



2. The musculature of the trunk is developed exclusively from 

 the cell -layer of the primitive segments that abuts upon the chorda 

 and neural tube, which by the formation of muscle-fibrilla? is con- 

 verted into a muscle-plate. 



3. The muscle-plate enlarges dorsally and ventrally, where it 

 becomes continuous (zone of growth) with the outer (lateral) epi- 

 thelial layer of the primitive segment, and spreads itself out over 

 the neural tube above and into the walls of the abdomen below. 



4. The original musculature consists of segments of longitudinal 

 fibres (myomeres), Avhich are separated from one another by connec- 

 tive-tissue partitions (ligamenta intermuscularia). 



5. The musculature causes the first segmentation of the body of 

 Vertebrates into equivalent successive parts or metamera. 



6. Buds grow out from the muscle-plates (Selachians) into the 

 fundaments of the limbs, and thus furnish the foundation for the 

 whole musculature of the extremities. 



7. In the head-region of Vertebrates the musculature is developed 

 not only out of the primitive segments, the number of which in 

 Selachians amounts to nine, but also out of that part of the middle 

 germ-layer which corresponds to the lateral plates of the trunk, and 

 which is divided up by the formation of the visceral clefts into sepa- 

 rate visceral-arch cords, which in the Selachians are provided with 

 cavities. 



