No. 3.] GROWTH AND METAMORPHOSIS OF TORN ARIA. 41 1 



contact with the ectoderm, being continuous, however, with the 

 two cells of the preceding section at their inner part. This 

 mass of cells forms the beginning of an important organ, which 

 I shall speak of as the proboscis vesicle, instead of the older 

 name, "heart." It is not possible to say with absolute certainty 

 from what layer these cells arise. They come neither from the 

 digestive tract, nor from the anterior enterocoel, but it is possi- 

 ble that they may come either from the ectoderm, or from 

 mesenchyme cells. Spengel described them as having an ecto- 

 dermic origin. On the contrary, all the evidence I can get 

 gives strong probability that they really come from one (or 

 more ?) mesenchyme cells which were applied to the inner sur- 

 face of the ectoderm. They are never, so far as I have seen, 

 even in their earliest stages, intimately fused with the ectoderm, 

 but lie applied to its inner surface, and if in reality they came 

 from a thickening of ectoderm at this place it could be easily 

 recognized. Again, as they lie on the inner surface of the 

 ectoderm, they strongly resemble the scattered mesenchyme 

 cells found in similar positions. The fate of these cells is 

 important, and we may now follow them through some of the 

 later stages. By increasing in number, they form a large and 

 solid mass of cells which pushes into the space between the 

 ectoderm and enterocoel, but retains for a while its contact at 

 one point with the ectoderm. An optical section, a side view 

 of the enterocoel, Fig. 15, shows this solid germ of the pro- 

 boscis vesicle. Later, the connection with the ectoderm is lost, 

 as the cell mass pushes nearer to the enterocoel, and at the 

 same time becomes rounded, and the cells arrange themselves 

 in columnar order around a small central cavity, as shown in 

 Fig. 16. This is also an optical section, but actual sections 

 were also cut to verify each of these stages. The anterior 

 enterocoel has also enlarged greatly during this period, and the 

 vesicle moves nearer and nearer to this enterocoel, until it comes 

 to lie in the corner where the duct of the enterocoel joins the 

 central cavity, also lying on the right side of this duct, as in 

 Fig. 17. Where the vesicle touches the wall of the enterocoel, 

 the latter pushes somewhat inwards, and a slight proliferation 

 of cells takes place. The vesicle lying against the walls of the 

 enterocoel does not apply itself entirely to it, but there remains 

 a space between the two, which remains a part of, and opens 



