DEVELOPMENT OF SPELERPES BILINEATUS 199 



herent property of the cells of the egg, especially the yolk-cells, 

 just as much as it is of ectoderm cells to form the neural struc- 

 tures. It explains nothing to make this statement, but it com- 

 pels us to look further, if we wish to understand the mechanics 

 of gastrulation. 



Thinning of blastopore roof. The roof of the blastopore con- 

 stantly becomes thinner from late cleavage until the end of gas- 

 trulation. The increase in size of the egg alone during this time 

 will not accommodate the increased surface provided by the thin- 

 ning ectoderm. Stains made on the upper surface of the egg 

 in its central half show a constant tendency to increase in area 

 by spreading centrifugally, due, probably, to a re-arrangement 

 of its cells, since stained spots near the equator become elongated 

 into definite bands, indicating a division of cells followed by migra- 

 tion of part of the daughter cells. ^ When the egg is largest, 

 the upper hemisphere, now translucent, slightly overhangs the 

 lower, that is, its radius is slightl}^ larger. The necessary surface 

 for this increase is, I believe, furnished by the thinning central 

 half of the upper hemisphere which forms approximately one- 

 fourth the entire surface of the egg. Occasionally, an egg fails 

 to gastrulate, but becomes very large. Such eggs have unusually 

 thin roofs. 



The thinning of the outer half of the upper hemisphere provides 

 the necessary ectoderm for the ventral hemisphere. As the 

 central half of the upper hemisphere, containing the small gran- 

 ules which are to form the embryo, remains within the upper 

 hemisphere, the embryonic material (in narrower sense) is not 

 carried below the equator, as in the frog. Consequently, the 

 embryo differentiates largely in the upper hemisphere. 



Very often the middle half of the upper hemisphere is trans- 

 lucent during some stages of gastrulation. In some cases a shad- 

 owy area can be seen projecting upwards above the blastopore 

 into the translucent area as described by Ikeda ('02) for Rhaco- 

 phorus. This shadowy area has been noted to increase. Its 

 shape is exactly what one would expect if it were the tongue of 

 cells about the archenteron, shown in figs. 30-33. But unfor- 



^ For method, etc., see page 209. 



