MUSEUM OF COMPARATIVE ZOOLOGY. 137 



Fig. 51, Plate VII. Figs. 63, 68) ; but that the new cells so formed do not 

 all remain in the brain is indicated by the fact that the brain does not in- 

 crease very rapidly in size. (Compare Plate III. Fig. 19, and Plate IX. 

 Fig. 77.) This rapid cell division would be inexplicable upon the as- 

 sumption of an origin in situ. Secondly, at an early stage the lopho- 

 phoric nerve is already seen extending from the brain to the adjacent 

 inner layer, with which it remains in contact. A longitudinal section 

 through the middle of this nerve shows a prolongation of the lumen of 

 the brain extending into it, so that its upper wall passes directly into the 

 upper wall of the brain, and its lower wall into the corresponding part 

 of the central organ (Plate YLL Fig. 68, lu. gn., n. loph.). The proxi- 

 mal part of the lophophoric nerve is thus to be regarded as a pocket of 

 the brain. The existing condition is not what we should expect if a 

 cord of cells derived from the outer layer of the lophophoric arm had 

 secondarily fused with the brain. Thirdly, I have never found any good 

 evidence that cells were being given off from the outer layer of the arm 

 at its tip to form the nerve, where we should look for such a process, if 

 anywhere ; on the contrary, the nerve is quite sharply marked off from 

 the outer layer at this point, as will be seen by reference to Figures 64- 

 67 (Plate VII.). These ligures represent successive transverse sections 

 from a young lophophore arm of about the stage of development of that 

 shown in Figure 71. Figures 65-67 were drawn from one arm in about 

 the position indicated by the lines 65-67 in Figure 71. Figure 64 was 

 drawn from the opposite arm of the same individual, and in about the 

 region of Figure 65. In Figures 64 and Q5 there is a small space be- 

 tween the nerve (n. loph.) and the overlying cells of the inner layer (*.). 

 This may be due to shrinkage, but in any event it indicates a complete 

 independence between the two cell masses which it separates. Over the 

 nerve the cells of the layer % are shorter than elsewhere. This might 

 be considered as an indication that the cells had recently divided in 

 order to give up cells to the nerve, which, on this assumption, would be 

 formed in situ. Three appearances, however, indicate that the cells of 

 the layer i. have been rather subjected to crowding at this point, as 

 though by a mass of cells forcing their way between them and the layer 

 ex., and gradually increasing in volume, (a.) The surface of the layer i. 

 is raised above the general level directly above the nerve, (b.) The cells 

 of the layer i. are somewhat broader over the nerve than elsewhere, and 

 the nuclei are shorter, but thicker. These are the conditions which we 

 should expect in an epithelium subjected to pressure by the intrusion 

 of a mass of cells at its base, for in volume the crowded cells compare 



