510 Howard Edwin Enders. 



four-celled stage j)asses into the eight-celled stage by division in a 

 horizontal plane. The lower cells are slightly larger than the upper 

 group, but in each group one cell is larger than its neighbors, (Mead 

 calls attention to the left-oblique cleavage in the eight-celled stage 

 of Chsetopterus). The subsequent divisions are nearly synchronous 

 in all the cells to the thirty-two-celled stage and their size, excepting 

 the D cells, is more nearly uniform than in the earlier stages, some 

 cells divide precociously as there is only a theoretical sixty-four- 

 celled stage. The embryo becomes elongated somewhat and over its 

 whole surface soon develop cilia that do not penetrate the fertili- 

 zation membrane. When the embryo is four hours of age the cilia 

 vibrate rapidly and cause it to rotate within the membrane. Al- 

 though Mead ('97) says the polar bodies are ingested by the rosette 

 cells in the sixty-four-celled stage, I saw the polar bodies attached 

 to the inner surface of the egg-membrane in which a larva was 

 rotating and from which it soon escaped when the wall was ruptured. 

 When five hours old the larva escapes from its fertilization membrane 

 and swims actively at the surface of the water by a rotation of the 

 body on its long axis. At this age the body is ovoid in form and is 

 covered with cilia of uniform length excepting at the broad anterior 

 end where there is a tuft of several longer ones. Eighteen hours 

 after fertilization the close union of the several elongate cilia, and 

 those of the posterior, narrower portion of the body become pro- 

 gressively larger backward. The body becomes more elongate and, 

 when from twenty-one to twenty-four hours of age, the mouth opens 

 at the ventral surface some distance in front of the mesotrochal band 

 of cilia ; and a pair of inconspicuous pigment-spots is now seen 

 widely separated on the dorsal side of the body and forward from 

 the mouth. "S^^ien the larva is twenty-four hours old the meso- 

 trochal band of cilia is stronger and back of it, on each side, is a 

 stout flagellum. The mouth, which appears as a triangular slit, 

 communicates wath the alimentary canal. It consists of a trans- 

 versely-directed esophagus which communicates with the dilated 

 stomach. The stomach opens into a short intestine which is sepa- 

 rated from it by a constriction of the walls. The anus is formed 

 on the dorsal side of the body just in front of the posteriorly-directed, 



