410 MORGAN. [Vol. V. 



side) and lateral region of the embryo. At one point, however, 

 in the region of the apical plate the two sides, right and left, 

 come into contact with one another, or, more exactly, they fuse 

 with the apical plate. This is shown in Fig. 2, and on an 

 enlarged scale in Fig. 4. The union, as the figures show, is so 

 close that I could not actually determine by observation that 

 the band on entering the apical plate emerged again on the 

 same side (right or left) of the embryo, to be continued forward 

 in the preoral region. However, when speaking of the later 

 stages of the embryo, I shall give my reasons for believing it 

 better to assume this the true course of the bands. The circular 

 band runs around the posterior end of the embryo and is seen 

 in the lower end of the figure, cb. In Fig. 4 there is seen, 

 right and left of the middle of the apical plate, the two eyes of 

 the Tornaria. The structure of these is better seen in older 

 stages and will be described later. Serial sections of this 

 embryo show that there is no trace as yet of the posterior enter- 

 occels, nor of the gill pouches. 



Fig. 5 is a drawing of an older stage. The course of the 

 ciliated bands is almost the same as in the last figures. The 

 anterior longitudinal band has a more sinuous course, fore- 

 shadowing the arrangement found in the later stages. The 

 circular band, cb, as before, runs around the posterior part of 

 the embryo. The water tube is seen in side view opening to 

 the exterior, and if the embryo be rolled over so that the dorsal 

 surface is turned upwards, there may be found in the region of 

 the external opening of the water pore, but beneath the surface, 

 a small solid mass of cells. It lies above and to the right side 

 of the opening of the enteroccel, but entirely separated from the 

 latter organ, and its position is indicated approximately by v in 

 Fig. 5. Two sections through this region are shown in PI. XXV., 

 Figs. 13 and 14. In Fig. 13 two large cells in close contact 

 with the thin ectoderm are seen. At each end they send out 

 processes which adhere closely to the inner surface of the 

 ectoderm. The ectoderm itself, in this region, is extremely 

 thin ; and at the place where the section is cut the surface is 

 folded a little, so that not only the cut end is exposed, but a 

 part of the inner surface as well, and it is on the latter that the 

 two cells are lying. The other section (Fig. 14) shows the 

 remaining cells of this thickening ; these are not in such close 



