Structure aud Development of the Nephridia. 355 



immediately anterior to the stomach. At thìs stage, the diaphragm 

 is incomplete dorsally aud its muscular fibres are as yet altogether 

 untbrmed. Large cells resembling in appearauce the mesenchyme 

 cells of earlier stages, are applied to its lateral regions and in the 

 ueighborhood of its jnnction with the Oesophagus. These cells evi- 

 dently play an important part in the formatiou of the diaphragmal 

 musele fibres, since the latter make their earliest appearance in 

 regions closely corresponding with the distribution of the cells. At 

 a stage of 12—13 somites, the diaphragm has increased considerably 

 in extent; and muscle-tìbres, which from the first show a continuity 

 with the longitudinal muscles of the body-wall, are being laid down 

 in its lateral regions. The extension across the body-cavity has in 

 most larvae become complete by the time of appearance of somite 

 XIV; and an uninterrupted partition, partly muscular, partly mem- 

 branous, is thus formed at the posterior boundary of the third somite. 



At this stage, the essential characteristics of the diaphragm are 

 briefly as follows: at its lateral Insertion it is entered on either 

 side by muscle-fibres , continuous with the longitudinal fibrils of 

 the body-wall; these extend completely across the diaphragm, pass- 

 ing below the Oesophagus and forming a broad band of muscular 

 tissue extendiug directly across the body-cavity from side to side. 

 Large nuclei, belonging to the cells above mentioned, are applied 

 to the diaphragm in the region of the muscle-fibres; at the Inser- 

 tion into the Oesophagus this organ is constricted, and the muscular 

 fibres and accompanying cells are especially well developed. In its 

 median dorsal and ventral regions, bowever, the diaphragm stili 

 remains membranous and non-muscular. 



It seems probable, from the appearauces presented at this and 

 later stages, that the cells applied to the originally membranous 

 diaphragm contribute to the formation of its muscle-fibres, i. e., act 

 as myoblast cells, resembling in this respect, the early mesoblast 

 cells of the growing region which give rise to the longitudinal 

 muscles of the body-wall (see below). At the same time the direct 

 continuity of the diaphragmal muscles with those of the body-wall 

 seems to indicate that fibres from the latter are being directly pro- 

 longed into the diaphragm by a process of ingrowth. The diaphragmal 

 muscles would on this view bave two sources; if, bowever, the 

 muscle-fibres of the body-wall also increase in number by the acti- 

 vity of myoblast cells (as seems probable], these two modes of origin 

 would be fundamentally identical in nature. 



