414 MONTG OMER Y. [Vol. XIII. 



lowering the tube of the microscope, i.e., with a focus a little 

 too high or too low, the enveloping sheath of Schwann is brought 

 into view, presenting the illusory appearance of a stained struc- 

 ture lying in the axis of the neurochord. It is possible that 

 Burger ('90b) had experienced such an optical illusion when he 

 described an irregular, staining structure lying in the axis of the 

 nerve tubule in C. marginatum ; otherwise it must have been an 

 artifact, as I found no positive evidences of such a structure 

 in C. lacteus. On cross section (Figs. 30, 40, Ax. CI. IV), 

 the neurochord is found to consist of an unstaining, homogene- 

 ous disc (the hyaloplasmic axis cylinder); this is immediately 

 bounded by a very fine spongioplasmic layer, and outside of the 

 latter is the thicker, granular sheath of Schwann, which stains 

 with eosin. I have been unable to find traces of a myelin 

 sheath ; but since the individual examined had been fixed in 

 50/0 alcoholic solution of sublimate, alcohol having the property 

 of dissolving myelin, the nemertean neurochord must be studied 

 after the action of a fixative containing OSO4 (this acid black- 

 ening myelin) before it can be determined whether such a 

 sheath is present or absent. And this point is worth investi- 

 gation with regard to the possible homologies of these neuro- 

 chords with those of the annelids, crustaceans, Amphioxus, 

 and even Myxine} 



The neurochord of cell IV divides dichotomously, though the 

 points at which it divides are situated at considerable distances 

 apart. Fig. 42 a, b shows such dichotomic divisions on longi- 

 tudinal sections. It is the rule that each of the branches has 

 one-half the diameter of the neurochord before the point of 

 division, so that it is a strictly equal dichotomy. In an excep- 

 tional case (Fig. 42 c) were two branches given off at the same 

 point from the one side of the neurochord, though the diameter 

 of the latter behind this point was not diminished ; this should, 

 perhaps, be regarded as a pathological case, and these two 

 branches would not correspond to collaterals, since Biirger ('9ib) 



1 H. Schultze ('79) first discovered myelin in the nerves of invertebrates within 

 the substance of the axis cylinder (between his " primitive fibrils ") in Unio and 

 Anodonta. Myelin sheaths of the nerve fibres of vertebrates have been described 

 by Leydig ('85), Retzius ('89b), Friedlander ('89), for annelids and crustaceans, 

 and by Apathy ('91) for ffirudinea. ^ 



