MUSEUxM OF COMPAliATIVE ZOOLOGY. 137 



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

 all remain iu the brain is iudicated by the fact that the braiudoes uot in- 

 crease very rapidly iu ssize. (Compare Plate 111. Fig. 19, and Plate IX. 

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

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

 plioric 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 tins 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 tlie central organ (Plate Vil. Fig. 68, Iu. gn., n. luph.). The proxi- 

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

 the brain. '1 he existing condition is not what we should expect if a 

 ct)rd of cells derived from the outer layer of the lophophoric arm had 

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

 evidonce that cells were being given ott 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 cpiite sharply marked off from 

 the outer layer at this pniut, as will be seen l)y reference to Figures 64— 

 67 (Plate Vll.). ' These ligures I'epresent successive transverse sections 

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

 shown in Figure 71. Figures 6.3-67 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 65 there is a small space be- 

 tween the nerve (ji. loph.) and the overlying cells of the inner layer (i.). 

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

 indepeiuleuce between the twt) cell masses which it separates. Over the 

 nerve the cells of the laver i are shorter than elsewhere. This mifrht 

 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 ])oint, as 

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

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

 is raised above the general level directly above the nerve. (/>.) The cells 

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

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

 should expect in an epithelium subjected to [)ressure by the intrusion 

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



