SEXUAL ORGANS AND THEIR PRODUCTS IN PHORONIS. 



Directly beneath that layer, and therefore interposed between it and the 

 connective-tissue layer, there remain at wide intervals small and flat 

 peritoneal cells which may be said to retain their primitive condition. 

 These small cells may best be observed on longitudinal sections of the 

 capillaries (Pl. V., fig. 2, /.p.). Irregularly scattered though they are, 

 they may perhaps without impropriety be considered to form a fourth 

 layer to the wall. Those cells are of great importance in that they give 

 rise to the germinal cells. Not that all the capillaries which later stand 

 in relation with the sexual glands undergo the above metamorphosis 

 nearly simultaneously, but there may remain for a considerable length of 

 time after some have completely gone through it, such others as are 

 or more less belated in the process or have not even begun to show it. 



A capillary after the completion of the change has increased in 

 external thickness, though the calibre of the lumen remains tha same as 

 before. Or, it may be more proper to say that the capillary is now in- 

 vested all around by a remarkably thickened peritoneum, consisting of 

 columnar or pyramidal cells arranged in a layer. A cross- section (Pl. V., 

 fig. 1) presents a wheel-like appearance. In the centre is the capillary 

 lumen, lined by the usual endothelium which rests on a connective-tissue 

 layer. Externally to this are arranged the large and tall peritoneal cells 

 in a radial manner. As the result of this mode of £ rangement, the 

 enlarged ce'ls are narrowed towards their inner end. The small perito 

 neal cells which should occur at their base are very difficult to observe 

 on cross-sections (and are therefore not shown in fig. 1). 



The enlarged peritoneal cells exhibit a fine but distinct bounding 

 membrane. The relatively small nucleus is always situated near their 

 outer and broader end — often almost in contact with the cell-membrane 

 at that end. The cytoplasm is of a clear appearance, being sparsely and 

 finely granular ; it is lightly coloured by staining reagents. In certain 

 preparations an indication of a reticular structure of the cytoplasm was 

 observable, reminding me of the " Fadennetzwerk " described by Cori 

 in the same cells of P. psavimophila ; but that is certainly not a con- 

 stant phenomenon in the species studied by me. 



