DEVELOPMENT OF CENTRIFUGED EGGS AND FRACTIONS I43 



and have developed into small normal plutei with short arms after 

 about three days, with normal Arbacia skeleton with a spiny base and 

 lattice-like arm skeleton but with short arms (Plate XII, Photograph 

 13). After about four days, pigment spots appear. These must be a 

 new formation in both the clear quarter and white-half plutei, since 

 the egg brought in no pigment. Many of the clear quarters develop 

 abnormally as loose clusters of cells caused by the thin hyaline layer, 

 and the nuclei often become very large, three times the normal dia- 

 meter. Many of the plutei are abnormal with abnormal skeleton. 

 There are no mitochondrial granules in the clear quarter-egg, and none 

 are brought in by the sperm in any appreciable amount, as shown by 

 staining with methyl green. The blastulae also have none, but the 

 plutei have them, localized especially in the cells around the gut, as 

 they are in plutei from whole eggs and white halves. These mitochon- 

 dria must be a new formation, just as are the pigment granules which 

 appear in the pluteus from the white half-egg. 



The slow development of the clear quarter, though having two 

 nuclei, argues against Whitaker's (1929) idea that the ratio of the 

 amount of nuclear and cytoplasmic material is the determining factor 

 in cleavage rate. 



When treated with a parthenogenetic agent, hypertonic sea water, 

 a fertilization membrane may be formed, and early nuclear changes 

 may occur, but no pluteus is formed as in the case of the white half. 



g. Mitochondrial (Granular) Quarter 



The mitochondrial or granular quarter (lower part of the white half) 

 contains all the mitochondria and some yolk and no nucleus. It may 

 cleave after fertilization quite regularly, and typical 2, 4, 16, etc. cell 

 stages have been obtained, but usually cleavage planes fail to come in, 

 so that multi-astral and multinucleate cells are common. The devel- 

 opment is a little slower than in normal eggs. Swimming blastulae have 

 been formed in 9 hours, but no skeleton and no plutei. 



h. Yolk Quarter 



The yolk quarter (upper part of the red half) contains only yolk 

 granules and no nucleus. It develops when fertilized, like the mitochon- 

 drial quarter with sometimes regular cleavages and sometimes it is 

 multinucleate, lacking cleavage planes. It develops further, however, 

 forming plutei with irregular skeletons and pigment, but no perfect 



