No. 3-] THE GASTRULATION OF AMPHIOXUS. 583 



yolk granules in the ectoderm are fainter and are beginning to 

 disappear. The granules grow fainter in that part of the ecto- 

 derm which corresponds to the animal pole. Around the blas- 

 topore they persist longer; the cells outside the dorsal lip 

 containing granules similar to those in the cells of the dorsal 

 wall of the archenteron. 



An older stage, two hours later than the last, is drawn in 

 PL XXXIII, Fig. 5. The embryo has greatly elongated, the 

 blastopore is reduced in size and now opens somewhat on the 

 dorsal surface at the posterior end of the body. The cells 

 over the dorsal wall of the archenteron are broader than those 

 elsewhere ; they are also clearer and contain fewer and lighter 

 yolk granules. As much as two-thirds of the dorsal wall is 

 formed by these cells, which pass gradually into the more 

 anterior cells containing more yolk. The cells of the ventral 

 wall are smaller than those at the anterior end and are also 

 smaller and darker than those over the dorsal wall. 



The cells of the ectoderm over the dorsal surface are very 

 similar to the endoderm cells over the dorsal wall. The more 

 anterior of those ectodermal cells are smaller, while those near 

 the dorsal lip of the blastopore are almost identical with those 

 within. 



On the ventral wall the ectodermal cells are smaller than else- 

 where, but they grow larger as they approach the ventral lip of 

 the blastopore. At the sides of the blastopore, also, there are larger 

 cells, so that a ring of large cells surrounds the blastopore. 



In the later stages the cells over the dorsal and dorso-lateral 

 walls of the archenteron continue to grow clearer and the yolk 

 in them to disappear. A cross-section of an embryo at 8 a.m. 

 is shown in PI. XXXIII, Fig. 6. Here the difference between 

 the dorsal and ventral wall is clearly seen. The dorsal surface 

 of the embryo is slightly flattened to form the medullary plate. 

 This section is taken at about the level of the posterior third 

 of the embryo. Sections of the same series taken at the 

 extreme anterior end show that the cells on the dorsal wall 

 contain more yolk and resemble those on the ventral wall. 



In the later stages changes take place in the endoderm, 

 bringing about a rearrangement of the cells and also involving 



