40 Edmund B. Wilson. 



by exactly vertical section passing through the axis and bisecting 

 the polar areas. 



{a) Fragments obtained by horizontal or oblique section. — 

 Under this heading may be grouped all fragments obtained by 

 sections passing in such a plane as to separate the polar areas, so 

 that one fragment contains only the upper, the other only the 

 lower, of these areas. These may be designated respectively as 

 the upper and the lower fragments. Before maturation I have 

 not found it possible to distinguish the upper from the lower 

 fragment; but as soon as the polar bodies form, the upper frag- 

 ment may be at once identified with certainty, since it alone pro- 

 duces these bodies. I have not thus far observed any difference 

 between the results of horizontal and oJ& oblique sections. 



The upper and lower fragments differ in a characteristic way, 

 both in the form of cleavage and In the structure of the resulting 

 larvae; though it should be added that this appears most clearly 

 In the cleavage-process, since many of the embryos die before 

 reaching the trochophore stage, and many of the remainder be- 

 come wholly monstrous in form. Nevertheless the main result Is 

 given with great consistency by a comparison of the larvae. This 

 contrast is especially striking when two fragments from the same 

 egg are compared; and within rather wide limits It Is Independent 

 of the plane of section and the size of the piece, certainly as far 

 as the form of cleavage Is concerned, and apparently also as re- 

 gards the larval type. Whether large or small the upper frag- 

 ment forms the polar bodies In normal fashion, and In many 

 cases segments in essentially the same way as an egg from which 

 the polar lobe has been removed. The first cleavage takes place 

 without the formation of a polar lobe and Is Invariably equal 

 (Figs. 53, 56, 62, etc.), and the same applies to the second cleav- 

 age. Frequently the two pairs of cells shift during or after the 

 second cleavage, so as to produce a "cross-form," the succeeding 

 divisions of which are difficult to analyze. In many cases, how- 

 ever, the four cells remain In .nearly the same plane ; and In such 

 cases the succeeding divisions conform to the regular rule of 

 spiral cleavage, quartets of micromeres being found by alternat- 

 ing dexlotropic and lelotropic divisions. (Fig. 69.) 



