MORPHOLOGY OF EYE MUSCLE NERVES 31 



and for this reason has been universally ignored in all drawings. 

 Paton ('07), for example, does not indicate its presence in the 

 sections figured by him (figs. 1 and 2, pi. 23) although it is un- 

 doubtedly present in those stages. It may not be confused with 

 the protoplasmic paths emphasized by him and Held ('06, '09). 



The plasmodesms of Held ('09) appear to be formed by the 

 union of the plasmoid material of the intercellular spaces with 

 the amoeboid protrusions of the basal cells of the germ layers. 

 At the stage represented in figure 1, protoplasmic processes 

 are lacking in the case of the neural tube, the outside boundary 

 of which consists of an imperforate basal membrane. The 

 chorda is likewise without protoplasmic outgrowths. The 

 median surface of the somite, however, shows a few short and 

 inconspicuous amoeboid extensions. However, between the 

 homogeneous, slightly stained protoplasmic outflows of the 

 somitic cells and the unstained, granular material of the vac- 

 uolated plasmoid material between the germ layers, there is 

 distinct contrast, although the two appear to connect with each 

 other. In the drawings the plasmoid substance is greatly 

 exaggerated in order to show it at all. As has been stated, 

 it has been generally overlooked in most embryological studies. 



Even when the films of plasmoid material coalesce as the 

 result of reagents, they require intense stains and special illu- 

 mination to make them visible. They form a most attenuated 

 material for the production of nerves. It would seem most 

 unlikely that growing nerves would trust themselves to such 

 flimsy paths as guides to their destination. That students of 

 nerve histogenesis should seriously consider such all-but-in- 

 visible films of non-protoplasmic material as the substance or 

 path of a growing nerve suggests that followers of the Hen sen 

 hypothesis are in desperate need of a material basis for their 

 assumptions. In no true sense do the plasmoid films constitute 

 a primary protoplasmic connection between tube and somite. 

 The actual protoplasmic connections are effected secondarily 

 as the following evidence shows. 



