MUSEUM OF COMPARATIVE ZOOLOGY. 113 



middle portion of the alimentary tract has been quite cut off from that 

 of the atrium by a constriction, the beginning of which is seen at ex., 

 Figure 24 (Plate IV.) and in a later stage at ex., Figure 28. The cells 

 of the outer layer are next pushed into the place of constriction and re- 

 move the alimentary tract at this point still further from the atrium, as 

 is shown in Figures 18 (Plate III.) and 28, ex. (Plate IV.). The error 

 of Nitsche is explainable on the ground that he believed the stage of 

 Figure 18 to be the earliest in the development of the alimentary tract. 



3. Origin of the Central Nervous System. — Metschnikoff ('71, p. 508) 

 first clearly recognized that the supra-cesophageal ganglion of Phylacto- 

 lsemata is derived from the inner cell-layer of the bud, — the same 

 layer which gives rise to the inner lining of the alimentary tract. 

 Nitsche ('75, pp. 359, 360) described and figured in an insufficient and 

 not wholly accurate way the process of the formation of this organ. 

 According to my observations, the central nervous system arises directly 

 over the middle of the horizontally placed alimentary tract in the posi- 

 tion marked gn. in Figure 18, Plate III. (compare also Figs. 17 and 28, 

 pam. gn.). The process by which the ganglion with its internal cavity 

 (Plate VIII. Fig. 73, lu. gn.) is formed will be more easily understood if 

 the reading of the text be accompanied by reference to the following sec- 

 tions. Figures 17, 18, 19, Plate III., and Figure 73, Plate VIII., show 

 successive stages in sagittal section. Figures 27-29, Plate IV., from a 

 single individual, are vertical right-and-left sections, the positions of 

 which are indicated by the lines 27, 28, 29 of Figure 17. Figures 30-32 

 are similar sections from an older individual (see lines 30, 31, and 32, 

 numbered at the lower border of Fig. 18), and Figures 33-38 are 

 from a still older polypide (compare lines 33-38, Fig. 19). By a study 

 of these sections, it is seen that the cells forming the floor of the brain, 

 pam. gn. y are derived from the inner layer of the bud, and indeed from 

 the very region of the layer which furnished cells to line the alimentary 

 tract (Plate II. Fig. 13, Plate IV. Figs. 25 and 24, ga.), and therefore 

 that the layer of cells forming the floor of the ganglion is directly con- 

 tinuous posteriorly through the anal opening (Plate II. Fig. 13, an.) 

 with the wall of the rectum, and anteriorly with the lining of the 

 oesophagus. The first marked differentiation of this region is effected 

 by the sinking of the centre of the floor of the neural tract (Fig. 18, gn.), 

 thus forming a shallow pit, which opens directly into the atrium above. 



The closure of the walls of the ganglion above must now be considered. 

 Concerning this process, Nitsche says : " Die Eander dieser Einstulpung 

 [my ' shallow pit '] wachsen nun wie die Eander der Medullarrinue 



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