434 TREADWELL. [Vol. XVII. 



quickly mapped out. The details of this process I am unable 

 to give, for it is impossible on surface views to see the outlines 

 of the cells ; and on account of the small size of the embryos, 

 sections are very unsatisfactory objects for study, the difficulties 

 in the way of accurate orientation being too great. I can only 

 say that the alimentary canal is rapidly formed, being practi- 

 cally completed by the twenty-third hour of development. As 

 in Nereis (Nos. 34, d and g), the posterior ectoderm cells bud 

 off some small cells, which enter into close relations with the 

 entodermal portion of 4d. Some stages in the invagination and 

 hollowing out of entoderm cells are shown in Text-Figs. 5, 6, 

 7, and 8. 



The stomodaeum, as already stated, is formed by cells from 

 both the second and third quartettes. The invagination which 

 some of these cells undergo is shown most clearly by the aid of 

 the landmark furnished by the cells 2b2.2.i.-,." already described 

 on p. 420. The anterior portion of the blastopore becomes the 

 mouth, and its sides are closed by cells from both the second 

 and third quartettes. The proctodaeum arises at the posterior 

 end of the blastopore, which I do not believe ever completely 

 closes. Thus both mouth and anus arise from it {cf. Conn, 

 No. 6). The cell Xi.2.2, as already stated, early invaginates to 

 form a part of the proctodaeal wall, but the exact origin of the 

 other cells which invaginate I was unable to determine. From 

 their position in earlier stages, I believe that the cells 3c 2+ and 

 3d2+ are the ones most deeply concerned, but the small size of 

 the cells and the shiftings of position which they undergo make 

 it impossible to be certain. (See PI. XXXIX, Fig. 46.) 



The trochophore is now fully formed. The apical tuft of 

 cilia still remains at the anterior end of the umbrella, and about 

 midway between this and the prototroch is a second tuft. (See 

 PI. XL, Fig. 60.) The prototroch is now a complete ring, the 

 origin of its cells having already been described. An adoral 

 zone of cilia extends from the mouth to the anus (" Neurotro- 

 choid " of Eisig), but no paratroch is present. On the ventral 

 surface of the umbrella are the two orange-colored "eye-spots," 

 and the large clear spaces, which are undoubtedly similar in 

 origin and function to the "frontal bodies" of Nereis and the 



