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CHAS. B. WILSON. 



groove deepens^ to the side of that blastomere which first 

 passes into the four- cell stage. After the blastomeres have 

 been appressed it remains a short distance from the bottom 

 of the groove. This distance increases with subsequent 

 segmentations, until at the close of segmentation it appears 

 near the centre of the external surface of one of the cells. 

 These changes of position without change of identity enable 

 us to determine — 



a. That the egg possesses no surface membrane, but has an 

 ectosarc similar to that of the amoeba. 



1). That the original ectosarc contributes but slightly to 

 the covering of the cleavage grooves, the larger portion of 

 that covering being a new structure. 



c. That after appression a portion of this new ectosarc 

 remains exposed along the lines of cleavage, and increases in 

 relative width as segmentation progresses. 



d. That gastrulation begins at the inferior pole, and the 

 axis of first cleavage becomes the future dorso-ventral axis 

 of the pilidium, and later of the adult (13). 



4. The blastomeres show vigorous spinning activities during 

 early segmentation. While the polar bodies are coming off, 

 and immediately after the separation of the second one, 

 numerous spin-threads appear on the surface of the egg just 

 beneath the bodies. A papilla is formed on either side of 

 the first cleavage plane, and from its summit spin-threads 

 radiate outwards, some of them connecting with those from 

 the poles of the first polar body, and re-establishing vital 

 connection between the two. Both papillae and spin-threads 

 disappear before segmentation begins. 



5. Papillae are also formed on the sides of the cleavage 

 grooves, from which spin-threads extend to adjacent blasto- 

 meres and aid in appression. These are easily seen in the 

 first groove, less easily in the second, and can hardly be 

 distinguished in the eight-cell stage. These threads are con- 

 tractile, and during the first cleavage, when the blastomeres 

 are almost separated, their contraction produces a perceptible 

 movement. 



