436 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



that the eight vegetative cells are divided into eight smaller, bordering the lower 

 tier of animal cells and not markedly different from them in size, and eight larger, 

 surrounding the vegetative pole. 



Though the orifice at the animal pole is always closed at this stage, that at the 

 opposite pole always remains open ; but a certain amount of rearrangement of the 

 cells about the pore takes place which, though very diverse in its manifestations, 

 always tends to narrow the opening, causing it to appear slit-like and forecasting 

 its eventual closure. 



The 48-cell stage, resulting in an almost perfectly spherical larva, in appear- 

 ance a blastula, is brought about by the division of the IG animal cells, which were 

 originally arranged in two superposed zones of eight each. The shifting about of 

 these cells before, during, and after this division causes great irregularity in the 

 position of the division planes of many of the individual cells ; but these division 

 planes can all be traced back to an originally meridional position. 



At this stage a rearrangement of the eight vegetative cells immediately sur- 

 rounding the apical orifice takes place, which results in its complete closure, and 

 also causes a slight flattening of the vegetative pole. 



The ensuing division of the vegetative cells gives rise to a 6-t-cell blastula, in 

 which the vegetative cells are noticeably larger than the others. 



As a result of the crowded conditions under which the embryos exist when 

 attached to the pinnules of the mother, combined with the mechanical weakness of 

 a 64-cell blastula, the larvae at this stage almost invariably depart slightly from the 

 perfectly spherical form, usually showing a more or less pronounced flattening 

 on one side. 



From this stage onward the cells do not divide with the previous regularity, 

 some of them outstripping the remainder in development. As a result of this a 

 regular 128-cell blastula rarely, if ever, occurs. 



As in other echinoderms, the cells of the blastula on commencing to divide 

 tend to assume a globular form ; toward the end of the division the daughter cells 

 again transform into longer prismatic cells. 



After division the chromatin content of the daughter nuclei is diminished, in- 

 creasing again at the inception of a new division. 



It is an interesting fact that throughout the greater part of the development the 

 embryos on a single mother are at any given time at approximately the same de- 

 velopmental stage. This is especially true in the early part of embryonal life. 



GASTBULATION. 



Flg-s. 1315, 1316, pi. 46. 



Gastrulation begins scarcely seven hours after the initial appearance of the 

 first cleavage furrow. A cell lying at the vegetative pole sinks inward on the 

 spot originally occupied by the cleavage pore; a neighboring cell does the same, 

 and others soon follow, so that there is formed at the vegetative pole a very 

 narrow, shallow, and slitlike furrow, which later becomes elongated and is 

 sometimes interrupted here and there, in length about equal to one-fourth of 



