120 CHAS. B. WILSON. 



meshwork of longitudinal and transverse fibres which cross 

 each other nearly at right angles. The longitudinal fibres 

 are then increased greatly in number and size, while the 

 transverse ones remain about the same. 



But the chief interest centres about the nervous system. 

 There has been a great deal of controversy over the origin of 

 this system in Nemerteans. Salensky in 1884 declared, as 

 the result of his researches on Monopora vivipara, that 

 the nervous system was derived from the epiblast (44). 



Hubrecht, two years later, declared that in Lineus ob- 

 scurus no portion of the central nervous system takes its 

 origin in epiblast, but that it is all of mesoblastic origin, and 

 he undertakes to disprove Salensky's statement in regard to 

 Monopora (26). 



Finally, Barrois and other investigators derive certain 

 important portions of the nervous system from invaginations 

 of the oesophagus, i. e. from hypoblast (7). All these inves- 

 tigators were working with embryonic material, in which it is 

 usually very difiicult to determine the exact relation of the 

 different parts. It is interesting to note in this dismembered 

 Nemertean the mode of regenerating the lateral nerve-cords, 

 since the process is attended by phenomena which leave no 

 possible doubt as to the origin of the new tissue. 



At the same time that the muscle-fibres first appear we 

 find upon the ventral surface two invaginations in the epi- 

 blast (fig. 21), forming two parallel grooves running longi- 

 tudinally along either side of the central line and quite near 

 to it. 



The ectoderm external to these grooves contains gland 

 cells, that in the grooves themselves and in the space 

 between them contains no gland cells, but seems to be made 

 up entirely of epiblast cells. A third groove then appears in 

 the centre of the space between the other two, dividing it 

 into two low ridges (fig. 23). Toward the inner surface of 

 each ridge the epiblast cells are gradually changed into 

 neuroblasts, and form one of the long nerve-cords. These 

 cells are not fully differentiated^ but seem more like the so- 



