224 Robert Thompson Young, 



or by the general parenchyma network. Whether certain parenchyma 

 cells themselves become modified to form neurons, or whether the 

 latter are entirely formed from the granular protoplasm laid down 

 by the parenchyma, is a doubtful point. The neurons are placed 

 mainly along the periphery of the cords and send their fibres into 

 the latter. There are no specialized "binding cells". The conducting 

 elements of the nervous system are the nerve fibrillae which enter 

 into close relation with the neuron nuclei and are probably continuous 

 therewith. These fibrillae are bound together into bundles — the 

 fibres, wliich later branch and anastomose with each other forming 

 a network, thru the meshes of which runs a second, connective 

 tissue, supporting network, which latter is comparatively poorly 

 developed. The fibres are formed from the granular neurogenetic 

 protoplasm. The neuro-muscular cells are developed in situ by the 

 modification of pre-existent parenchyma cells. 



g) The Sub-cuticula. 



Like the other tissues of the larva, the sub-cuticula is differ- 

 entiated out of the original parenchyma. In the embryonic condition, 

 both in the young bladder worm and in the bladder wall of the 

 older larva, the sub-cuticular cells are indistinguishable from the 

 rest of the parenchyma and are in union with the, as yet, in- 

 completely differentiated sub-cuticular musiîles and their neuro- 

 muscular cells, the ciiticula and the underlying parenchyma cells. 

 During the differentiation of the muscles and their consequent 

 separation from the parenchyma network, the connection between 

 sub-cuticular cells and muscles is lost. 



The various stages in the development of these cells may be 

 readily followed in passing from the bladder wall of a well developed 

 larva to the neck region. Fig. 51 represents a part of the wall in 

 the former region and Figs. 45 and 50 a few sub-cuticular cells 

 and a "mj^oblast" from the latter. In the first figure are seen 

 the sub-cuticular cells forming a syncytium of irregularly^ branched 

 elements which vary greatly both in shape and size. Not all of 

 these cells will form sub-cuticular cells, some of them retaining 

 their original parenchymatous nature, but it is impossible to 

 differentiate between them at this stage. Thej' are 

 all similar in appearance. Accompanjing the increase in 

 size of the cells, they show a gradual orientation, their long axes 

 placing themselves perpendicular to the surface of the bladder wall. 



