434 BXJLLETIN 82, UNITED STATES NATIONAL. MUSEUM. 



Twenty minutes after the inception of the first division a part of the eggs have 

 already divided into the four-cell stage, the remainder soon following. The second 

 division runs meridionally like the first, and at right angles to it, so that four cells 

 of equal size are formed with between them a free canal running along the axis 

 passing through the animal and vegetative poles, the first beginnings of the cleav- 

 age cavity. Each of the four cells thus rests upon the two neighboring cells, but 

 not upon the opposite cell. 



At the funnel-like opening of the central canal at the animal pole one, or even 

 two, polar bodies may often be observed. 



The third cleavage plane runs parallel to the equator, though not equatoriaUy, 

 for it lies nearer the animal than the vegetative pole and thus divides the embryo 

 into four smaller and four larger cells, which are directly superimposed. 



As in succeeding stages the cleavage cavity is now smaller at the animal pole, 

 where it is surrounded by the smaller cells, than at the vegetative pole, where it 

 appears between the larger cells. 



The embryo is now quadrilateral; but a definite relation between these quad- 

 rants and the body regions in the later embryo has not been determined. 



The next division first begins at the animal pole and divides the four small cells 

 into eight of approximately equal size. The division of these four cells is usually 

 not everywhere entirely synchronous. 



As a rule the furrows first appear on the four vegetative cells after all four 

 of the animal cells are entirely divided; as in the case of the cells at the animal 

 pole, the division of the vegetative cells is not always synchronous. 



As a result of these divisions the inner borders of the eight cells of the animal 

 pole retreat a little from the major axis, so that the cleavage cavity becomes wider, 

 while at the vegetative pole no change takes place. 



The following division rvms equatoriaUy and divides the eight smaller cells at 

 the animal pole into 16 of approximately equal size. The embryo is thus composed 

 of 24 cells, arranged in three superposed zones. 



After the division of the animal cells is completed, indeed often while it is 

 still in progress, the newly formed blastomeres rearrange themselves in such a way 

 as to bring about the complete closure of the orifice at the animal pole. This is 

 accomplished rather quickly, often before the end of the 24-cell stage; before the 

 completion of the 32-cell stage the orifice is invariable closed. 



This rearrangement of the cells at the animal pole does not always take place in 

 exactly tlie same way. Sometimes one of the small cells moves out of the ring 

 surrounding the opening more or less completely into the line of the major axis, 

 while again all eight of the cells about the opening may move toward each other in 

 such a way as to close it. The examination of a large number of embryos at this 

 stage shows very diverse conditions at the animal pole, making it clear that the 

 rearrangement of the cells is brought about in various ways. 



The 32-cell stage following arises from the 24-cell stage through the appear- 

 ance of furrows, parallel to the equator, which divide the eight vegetative cells. 

 These furrows do not always appear simultaneously on all the cells, and they are 

 not equidistant between the equator and the pole, but slightly nearer the former, so 



