DEVELOPMENT OF CENTRIFUGED EGGS AND FRACTIONS 1 39 



if the membranes are removed by drawing up the eggs in a fine capil- 

 lary, the eggs retain their aspherical shape (and develop). 



Cytological details can be observed in the living elongate egg: the 

 sperm aster, union of the pronuclei, enlargement and breaking of the 

 nucleus, cleavage spindle, can all be observed, except that astral 

 radiations cannot be seen unless there are granules. The spindle forms 

 in the long axis of the egg where there is room for it. The first cleavage 

 plane almost always comes across the short axis of the egg, parallel 

 with the stratification but not at the narrowest part, dividing the egg 

 into a smaller clear cell and a larger granular cell. This is in contrast 

 to the first cleavage of the spherical centrifuged egg described, where 

 the first cleavage plane is usually perpendicular to the stratification. 

 The shape of the egg determines the position of the first cleavage plane. 

 The second cleavage plane usually comes in perpendicular to the first 

 in the clear cell, and either perpendicular or parallel with the first 

 in the granular cell. The second and following cleavages are asyn- 

 chronous, the smaller clearer cells dividing more rapidly than the 

 larger pigmented cells. A slipper-shaped blastula is formed with un- 

 equal-sized cells. The pigmented area remains distinct through the 

 cleavages and in the early pluteus as in the spherical egg. 



Micromeres may be formed in any region of the elongate egg, even 

 at the oil cap (E. B. H.) (Plate VIII, Photographs 5, 6). They probably 

 form in relation to the original axis of the egg. They may be pigmented 

 but usually are not. 



c. White Half-Eggs. Plate IX 



The white half-egg which has been allowed to stand in sea water after 

 centrifuging, and become spherical, develops after fertilization, exactly 

 like the whole uncentrifuged egg cleaving at the same rate, sometimes a 

 Httle faster (E. B. Harvey, 1932, 1940c, 1 951). The nuclear phenomena 

 accompanying fertilization can be observed in the living egg with 

 great clearness, and they parallel those described by Wilson (1895, 

 The Atlas of Fertilization) in the unpigmented tgg oi Lytechinus (Toxo- 

 pneustes) variegatus. In the clear, nongranular portion of the egg, how- 

 ever, no astral rays can be seen. The first cleavage plane usually comes 

 in through the oil cap, but may be parallel with the stratification, or 

 at an angle. Following cleavages come in quite normally, but micro- 

 meres have not been observed. Blastulae are often formed, normal 

 except for size and color, and these may develop into plutei with well 

 developed arms and normal skeleton but more slowly. In many of the 

 cultures, however, the white halves develop abnormally, and especially 



