144 THE AMERICAN ARBACIA 



plutei have been obtained. There is more of the ground substance in 

 these quarters than in the mitochondrial quarters, as may be ascer- 

 tained by the clear layer often seen at the centripetal pole when the 

 red half is recentrifuged (Plate XIII, Photograph 5). This quarter is 

 also considerably larger than the mitochondrial quarter. 



i. Pigment Quarter 



The pigment quarter, containing all the pigment and a little yolk, is 

 very much smaller than any of the other fractions, only one twelfth the 

 volume of the whole egg. It develops after fertilization like the other 

 granular quarters, usually with multinucleate cells, though some regular 

 2-, 4-, 8-, 16 cell stages have been found. The development is very 

 much delayed. Some swimmers were found after a day, but there was 

 no further development. It is of interest that Morgan (1893) found that 

 fragments of the Arbacia egg 1/70 the volume of the egg would cleave. 

 These fragments were obtained by violent shaking of the whole eggs. 

 According to Tennent, Taylor, and Whitaker (1929) for Lytechinus 

 variegatus, fragments i /50 the volume of the whole egg could not be 

 fertilized, fragments 1/35 the volume could be fertilized but segmented 

 irregularly, 1/24 gave regular cleavage, 1/17. formed blastulae with 

 mesenchyme, i/io formed gastrulae and fragments about 1/4 the 

 volume of the whole egg formed plutei. 



j. Eggs Fertilized, Then Centrifuged 



When the whole egg is fertilized and then centrifuged (but not elongat- 

 ed) at any stage after fertilization, it may develop quite normally, 

 and give rise to plutei normal in every respect except for pigmentation. 

 The first cleavage plane usually passes through the oil cap, perpendicul- 

 ar to the stratification but may be parallel with it (E. B. Harvey, 

 1934). If centrifuged after the spindle has formed, this is thrown to 

 the centripetal pole, and the cleavage plane comes in through the 

 oil. By observing with the centrifuge microscope, one can see the 

 cleavage plane coming in during rotation at 6,000 X g (E. B. Harvey, 

 1933 b). Spooner (1909) obtained the same percent of development, 

 no matter at what stage after fertilization the eggs were centrifuged. 



When the fertilized eggs are broken apart after fertilization, at any 

 stage, the white half, containing the combined (^ and $ nuclei, may 

 cleave and develop quite normally through the blastula stage. A few 

 acquire a skeleton and later, pigment, and may become very abnormal 



