Structure and Development of tlie Nephridia. 3g3 



At tbe earliest appearanee the fibres have in cross section the 

 appearauce of a row of deeply-staining- dots at the boundaiy between 

 ectoderm and mesoderm (Piate 23, Figs. 13, 21 , 24). At a slightly 

 hiter stage (Fig-s. 15, 23) the dots have become larger and better 

 defined, and have assumed the typical appearanee of triangulär 

 wedge-shaped structures, whieh stain very deeply in the erythrosine 

 and project into the protoplasm of the mesoblast cells. In the ventro- 

 lateral region the mesoblast nuclei have become arranged in a single 

 layer, and in cross section the mesoblast has assumed the appearanee, 

 which is very typical for this stage, of a row of nucleated cells, 

 uuseparated by cell-walls, and containiug in their interior deeply 

 staining longitudinal muscle übrils Piate 23, Figs. 13, 15, 19, 23). 

 It is evident that the longitudinal muscle-fibres are origiually laid 

 down in the interior of these cells, which may therefore with pro- 

 priety be termed myoblast cells. The cells, however, it is to be 

 noted, preseut no Constant and regulär arrangement and are not 

 sharply defined from one another. At this stage the longitudinal 

 muscles are thus composed of fibrillae applied to the body-wall and 

 covered internally by a nucleated layer of undifferentiated protoplasm 

 (sarcoplasm). The condition recalls that which exists throughout 

 life in certaiu other species, particularly the Nemathelminthes. The 

 trausverse mascles on the other band are not visible uutil a much 

 later stage; they appear externally to the longitudinal muscles, and 

 apparently in relation to the inner layer of the ectodermal cells'. 

 The above myoblasts at all events have uothing to do with their 

 formation. 



Cross sections passing through the region immediately behind 

 the early septum show the above-described appearances in a charac- 

 teristic manner (Figs. 13, 15, 19, 23). Longitudinal sections (Plates 22 and 

 23, Figs. 9, 10 and 31), passing through the ventro-lateral regions, show 

 that the mesoblastic layer is thickest and best defined at the junction 

 of septum and body-wall. It is in this region that the embryonic 

 riidiments of the nephridia {n) first become distinguishable from the 

 rest of the mesoblastic layer, It must be emphasized however, that 

 there is at first no setting aside of special nephridial cells ; nothing 

 comparable to the septal nephroblasts or "funnel-cells" described by 



^ Ed. Meyer (1901) gives a summary of the evidence on this point, frora 

 which it would appear that au ectodermal origin is the mie, if not invariable, 

 lor the circular body-musculature of annelids. 



