21G PROCEEDINGS OF THE ACADEMY OF [Feb., 



disappears completely before the adult pharynx begins to form, but 

 its relation to that pharynx appears to be the same as in P. maculata, 

 as recently described by Curtis ('02). In fig. 45 the ventral side of 

 the embryonic layer is easily distinguishable from the dorsal side by 

 its greater thickness, and the degenerating pharynx (p^) is on the dorsal 

 side, as in Curtis's fig. 51, PI. 17. The embryonic pharpix disappears 

 so early, when many of the embryos are quite irregular in form, that 

 it is impossible to tell whether it has a fixed position relative to the 

 permanent pharynx or not, but my impression is that its position is 

 variable. There is no evidence whatever that the embryonic pharjmx 

 serves as a tube leading to the anlage of the permanent pharynx, as 

 described by Metsclinikoff ('83) for Planaria polychroa. 



Fig. 47 is from a 4-day embryo in which the pharynx-chamber 

 appears as a split in the thickened ventral region of the embryo. In 

 this stage pigment and rhabdites have begun to appear in the surface 

 epithelium-cells, and rhabdite-cells are found among the embryonic 

 cells, which are no longer like the early blastomeres, but closely resem- 

 ble the embryonic cells of newly regenerated regions of adult planarians. 

 Figs. 48 and 49 are sections of an older embryo (5-6 daj^s), showing 

 the permanent ectoderm well developed and full of pigment and 

 rhabdite-cells. So far as I am able to determine, the ectoderm is 

 formed from the outer embryonic cells and not by division of the earlier 

 scattered epithelium-cells. There is considerable evidence that rhab- 

 dite-cells migrate from the interior to the surface and become a part of 

 the ectoderm. In this embryo (figs. 48-51) the pharjTix (p^) is quite 

 large and has a lumen connected with the central yolk-area (fig. 50). 

 The yolk-area is being gradually divided up by strands of cells extend- 

 ing inward from the surface layer of embryonic cells to form the bound- 

 aries of the axial gut and its principal branches. Fig. 52 is from an 

 older embryo (7 or 8 days), in which the development of the digestive 

 tract is quite far advanced. In fig. 49 there is a section of a very young 

 eye (e), the pigmented cup consisting of only 5 or 6 cells. No brain 

 is yet distinguishable, but the lateral nerve-cords are represented by a 

 few strands of nerve -fibers (?i). In fig. 52 the e3'e is much further 

 advanced and the nerve-cords are larger. The eyes in all embryos of 

 this age are situated much deeper in the tissue than in the adult. 

 There is as yet no definite endoderm, but here and there are cells with 

 nuclei like those of adult endoderm-cells, and processes extend out 

 from them among the yolk-cells as seen in fig. 51, e. 



Fig. 53 is a cross-section of an embryo just before hatching (12th 

 dav). The lumen of the digestive tract is still full of yolk-cells and 



