DEVELOPMENT OF PARAVORTEX GEMELLIPARA 521 



tudinal muscle fibers which soon become so conspicuous in this 

 position (fig. 40). These sub-epitheHal muscle cells of the adult 

 Rhabdocoeles have been observed by many investigators. The 

 figures in Bronn's ''Klassen und Ordnungen" show the remarkable 

 extent to which they become fibrillated. 



In Paravortex as in most of the Rhabdocoeles, a muscle net 

 is developed just beneath the epidermis. Figures 40 and 41 

 show the outer layer of circular and inner layer of longitudinal 

 fibers in the adult condition. Since the figures in the various 

 works which have been accessible never show nuclei in con- 

 nection with the fibers of this muscle net, a vigorous attempt was 

 made to find the cells from which these fibers arise in the embryo 

 of Paravortex gemellipara. Of interest in this regard is the 

 cell shown in figure 39, for it is possible to trace several long 

 delicate fibers outward from the cytoplasm surrounding its 

 nucleus. The latter lie close beneath the external epithelial 

 cells in the position later occupied by the muscle net. Coe ('99) 

 'traced such a development of the muscle fibers in the Nemerteans 

 Micrura caeca and Cerebratulus marginatus. 



If this interpretation of the origin of the muscle fibers be cor- 

 rect, then a further question presents itself. Do these fibers later 

 become independent or do they maintain a connection with 

 such a cell body as is seen in figure 39? It is difficult to conceive, 

 judging from the parallel arrangement of muscle strands in the 

 adult, that the fibers are still in connection with these cell bodies. 

 The latter, moreover, cannot be detected in the adult tissue. 

 The conclusion is, therefore, that either the fibers become sepa- 

 rated from the nuclei, or that the latter by repeated division are 

 so reduced in size that they escape observation. 



e. Sex organs and vitellaria. Not until after the young leave 

 the mother's body does the true differentiation of the sex organs 

 commence. At the time the young worm breaks out of the cap- 

 sule and enters the surrounding mesenchyme the anlagen of the 

 two hermaphroditic glands are only loose masses of cells, one 

 on either side just lateral and posterior to the pharynx. In 

 one animal which had forced its way into the mother's intestine 

 a few cells of these paired anlagen were undergoing mitosis. 



