pluteus. 7. Anal or ventral view of pluteus. 8. Oral or dorsal view of 

 the same pluteus. 9. Side view of pluteus. 



PLATE V. 



Stained sections of developing eggs, fixed in Bouin, stained with 

 Heidenhain's haematoxylin. Photograph 1. Unfertilized egg. 2. Sperm 

 and sperm aster. 3. Sperm nucleus fused with female nucleus. 4. 

 "Streak" stage, with enlarged nucleus. 5. Prophase. 6. Metaphase. 

 7. Anaphase. 8. Telophase. 9, 10. Second cleavage. 11. Four cells. 

 12. Early blastula; note that the spindles have no asters. Magnified 

 about 450 X. 



PLATE VI. 



Metamorphosis. All photographs (except 12) are to the same scale 

 magnified about 24 X. Photograph 1 is of eggs at this magnification. 

 2. Three day plutei. 3. A new pair of arms with red tips forms after 

 11 days if the plutei are fed. 4. Another pair of arms, median, forais 

 when about 3 weeks old. 5. Arms longer; four weeks old. 6. Arms 

 begin to degenerate; four tubular processes, the auricles, are present; 

 5 weeks old; the body of the adult grows inside the pluteus. 7. Five 

 tube feet are formed in a sort of pocket; 2 months old. 8 to 10. Fifteen 

 primitive spines, three adjacent to each tube foot; 2/2 to 3 months. 

 11. More tube feet have formed; 3/2 months, the oldest raised in the 

 laboratory; diameter 1 mm. including spines. 12. Smallest young Ar- 

 bacia brought from the sea, about 6 mm. including spines. (Biol. Bull. 

 97:292, 1949, modified.) 



PLATE VIL 



Breaking of the unfertilized egg by centrifugal force, 10,000 X g. 

 Photograph 1. Stratified egg showing oil, clear layer, mitochondria, 

 yolk and pigment. The nucleus is always in the clear layer under the 

 oil cap. 2. White half, containing oil, clear layer, mitochondria, a little 

 yolk and the nucleus. 3. White half, with further centrifuging. 4. Clear 

 quarter, containing oil, clear layer and nucleus. 5. Mitochondrial or 

 granular quarter, containing all the mitochondria and a little yolk. 

 6. Red half, containing yolk and pigment. 7. Red half with further 

 centrifuging. 8. Yolk quarter, containing yolk. 9. Pigment quarter, 

 containing a little yolk and all the pigment. 10. Eggs in three layers 

 in sugar solution after centrifuging; white halves (top), unbroken 

 whole eggs, and red halves (bottom). 11. Unfertilized eggs stratifying 

 and breaking apart in the centrifuge microscope; photographed while 

 rotating about 6,000 X g. 12. White halves, taken from the centrifuge 



