IO2 BIOLOGICAL LECTURES. 



paratroch is differentiated, the embryo of AmpJiitrite contains 

 about two hundred cells, and the mesoblast bands are composed 

 of four cells each. At this stage the large invaginated ento- 

 derm cells are still solid, and have not formed the cavity of the 

 alimentary canal, and the large prototroch cells are very promi- 

 nent on the surface of the larva. 



In general, the development of Podarke through these 

 later stages agrees with Amphitrite, but from the standpoint 

 of strictly cellular development important differences appear. 

 The prototroch is at first completed by one " intermediate " 

 cell (i.e., one of the cells which originally lay between the arms 

 of the cross), and later two more cells come into it from the 

 lower hemisphere. These latter acquire their cilia very late, 

 after the trochophore is fully formed. Cells from the upper 

 hemisphere also migrate through the dorsal break in the proto- 

 troch, but instead of giving rise to a very small portion of the 

 subumbrella ectoderm fully three-fourths of this latter arises 

 from them. The descendants of 2d form a narrow band of 

 ectoderm around the proctodaeum, and some of them pass into 

 the wall of the latter. In Fig. 12 the portion of the trocho- 

 phore behind the dotted line, x, x, is the only part whose ecto- 

 derm is formed by the X cells. I have been unable to discover 

 any trace of a paratroch. 



In Podarke, also, immediately after the formation of the fifth 

 quartette of micromeres, which happens about ten hours after 

 fertilization, the entoderm plate invaginates and rapidly gives 

 rise to the alimentary canal, which is pretty definitely formed 

 by the twentieth hour, and the trochophore must begin to feed 

 very soon after this time. In Amphitrite, on the other hand, if 

 I understand Dr. Mead correctly, the alimentary canal is formed 

 much later, about sixty hours, and considerably later the 

 embryos settle down to the bottom and begin to feed. While, 

 too, there is a similarity in the method of formation of certain 

 organs, there is a marked difference, becoming more and more 

 noticeable as the development progresses, in the relative rate 

 of development of different parts. The early stages of Podarke 

 and of Amphitrite are passed through in about equal spaces of 

 time, and both begin to swim immediately after reaching the 



