568 Stewarts Paton 



the probability of the i)assage of these struetures from one cell to 

 another, but it is extremely difficult to find two nuclei with their 

 ueuroplasm lyiug in the same plane, so that the coutinuity may, 

 without doubt, be established. In Fig-. 14 it may be seen very 

 elearly that the continuity between thc two cells is established by 

 a network made up of coarse and fine Strands, and a similar 

 arrangement is noted in Fig. 15. The network in Fig. 14 is seen 

 to extend laterally, partially coveriug a third nucleus at the dorsal 

 margin of the central canal. Niimbers of sniall black granules are 

 visible at the various points of junction formed by the meshes of 

 the network, and these represent probably the cross sections of 

 fibrils running at right angles to the plane of section. Careful 

 study, with the aid of high power lenses and brilliant Illumination 

 demonstrates, that the fine and coarse Strands in the reticulum unite 

 so as to form a single net, and are not two distinct struetures. This 

 network or neurospongium is absolutely distinct from the coarse 

 faintly stained structure sometimes seen with in the spongioblasts. At 

 these early periods in the development of the embryo a network 

 of this character is only found at the poiut where the differentiation 

 of the neuroblasts proceeds with the greatest rapidity, and only in 

 or around the cells of this type. The objection that may possibly 

 be entertained by some, to the effect that at least a part of these 

 fibrils are derived from spongioblasts cannot be accepted as valid 

 as there is no evidence in favor of this view. In the large cells 

 of Beard the centre of differentiation of the protoplasm into fibrils 

 seems to be within the cells, but in the case of the ventral root,. as 

 already indieated, it is either in the plasmodesmata outside of the 

 cord or just within the outer triangulär zone or the narrow marginai 

 rim of protoplasm between the ventral and dorsal roots. In the 

 case of the spongioblasts the differentiation of these cells is in the 

 opposite direction, beginning in the inner row of cells dose to the 

 centrai canal. In well stained seetions there canuot exist the 

 slightest difficulty in distiuguishing the neurofibrils from the products 

 formed by the spongioblasts. There is no reason for supposing 

 that filaments from the latter, even if they were produced at this 

 time, may creep into the body of the neuroblasts along the arms 

 which occasionally unite these two different types of elements. At 

 the moment when neurotìbrillation begins neither the apical processes, 

 bodies nor nuclei of the spongioblasts contain any fibrils, and ali the 

 evidence points to the early atrophy and disappearance of these 



