412 MORGAN. [Vol. V. 



below into, the general blastocoel space. Lastly, the stage 

 of development is reached, shown in Fig. 18. The larva has 

 reached the stage figured in Fig. 6. The upper part of the 

 wall of the anterior enteroccel has thickened greatly ; not over 

 the whole extent, however, but more in the shape of a ring. 

 The transparent structure is seen from the right side. At 

 the lowest level of the figure, or on the side away from the 

 observer, the duct of the enteroccel is seen opening into a horn- 

 like prolongation of the enteroccel. Above this is seen the 

 large proboscis vesicle, which is applied closely to the walls of 

 the enteroccel. At the point of contact, the wall of the enter- 

 occel projects to some extent into its own cavity, and at this 

 point the lining cells proliferate into the lumen. At the level 

 nearest to the observer lies the right horn of the enteroccel. 

 Fig. 19 is a longitudinal section through Fig. 18 in the plane 

 of the paper and on the middle line (but turned in the opposite 

 direction). Above is the cavity of the enteroccel with its large 

 cavity and its walls thickened at two opposite points where the 

 section cuts through the thickened ring of cells. The upper- 

 most part of the enteroccel does not as yet have its walls 

 thickened to any extent. Just below the enteroccel is a cavity 

 opening into the general blastocoel space. It is roofed over by 

 the floor of the enteroccel, and below is bounded by the wall 

 of the proboscis vesicle (v). This is the cavity of the "heart," 

 or proboscis blood-vessel, which is filled with non-corpusculated 

 fluid, and is seen to. be merely a part of the general blastocoel 

 cavity partially surrounded by the walls of the enteroccel and 

 proboscis vescicle. The upper walls of the latter, forming 

 the lower and posterior boundary of the "heart" cavity, are 

 thickened, the cells seeming to be somewhat muscular. 



The series of figures represented by 20, 21, 22 are from 

 sections cut at right angles to Fig. 17, and shown by the letters 

 a, b, c. Fig. 20 (a) passes across the enteroccel, showing the 

 ring cut in two places. At the lower part of the figure appears 

 the cavity of the vesicle (v) with its walls applied closely to the 

 walls of the enteroccel. Between the two lies a small cavity 

 filled with blood (a), which is the heart cavity, or proboscis 

 blood-vessel. Only part of the next section is shown in Fig. 

 21 (b), which represents parts of the enteroccel, the heart, and 

 the vesicle. The walls of the latter are nearer together, and the 



