vol. ij Ritter-Davis.—Enteropneusta. 183 



relatively few cells. Plate XVIII, Fig. .">, shows a portion of the 

 ectoderm of both the oral and the extra-oral regions at the climax 

 of larval life. The extra-oral, it will be observed, contains a 

 large number of nuclei closely crowded and occupying the entire 

 thickness of the layer. In the flattening out and thickening up 

 of the ectoderm that takes place in metamorphosis, what happens? 

 Does the thin oral portion as well as the extra-oral contribute to 

 the ectoderm of the proboscis of the adult? If so the oral must, 

 obviously, undergo much greater histological transformation than 

 does the extra-oral. 



Speugel believed that the extra-oral, or tentacular field, gives 

 origin to the whole of the proboscis ectoderm, and that the oral 

 field ectoderm undergoes degeneration and resorption. We have 

 examined this point both on living animals and on sections, and 

 are convinced that in this species, at least, the ectoderm of the 

 oral field does not degenerate, but on the contrary thickens and 

 becomes ciliated to form its share of the ectoderm of the adult 

 proboscis. The series of figures, PI. XVIII, Fig. 8, and PL 

 XVII, Figs. 6 and 7, illustrate this. Figure 8 is from a larva at 

 the climax of development and shows at ec. or. the thin ectoderm 

 of the oral field with its flattened nuclei far apart; at ec. ex.-o 

 some of the thicker extra-oral ectoderm with nuclei much more 

 numerous; at c. r. a ciliated ridge with its closely packed, deeply 

 stained nuclei; and at lent, a tentacle with the ciliary bands on 

 its opposite sides. Plate XVII, Fig. (i is from another larva 

 at the very beginning of metamorphosis. Here it will be observed 

 that the ectoderm of the oral field, ec. or., is somewhat thicker, 

 and that the nuclei are nearer together. This structure is now 

 characteristic of most of the ectoderm of this field. Plate XVII, 

 Fig. 7 is from a larva well on the way to metamorphosis. The 

 tentacles are entirely gone, the musculature of the proboscis has 

 reached an advanced stage of differentiation, and the whole ecto- 

 derm is much thickened. At ec. or. is a section of the oral 

 ectoderm with remnants of the thicker ciliary ridges, c. r., on 

 the two sides. Not only, will it be observed, are the nuclei round 

 and numerous here, but this ectoderm as well as the adjacent 

 extra-oral ectoderm, has taken on the uniform ciliation which 

 characterizes the whole ectoderm of the proboscis of the adult. 



