142 THE AMERICAN ARBACIA 



f. Clear Quarter. Plate XII 



The clear quarter can be obtained by centrifuging the white half-egg 

 for 20-30 minutes at 10,000 X g; to break the whole egg into halves 

 takes only about 4 minutes. It is best to remove the white halves from 

 the first centrifuging and recentrifuge in a fresh graded sugar-sea water 

 mixture of the same density as the halves. The clear quarters are easy 

 to distinguish from the white halves in the one and two-cell stages, but 

 later it is difficult to distinguish them; they should therefore be segre- 

 gated from the white halves. The clear quarter has an oil cap but no 

 granules; it contains the bulk of the ground substance or matrix (about 

 two-thirds) . The total liquid in the egg has been computed as 61.1% of 

 the total volume (E.N.Harvey, 1932a). The clear quarter is about as 

 near pure protoplasm, without granular inclusions, as it is possible to 

 obtain in the living condition. Although the clear quarter-egg is, in the 

 living condition, free of the usual granules, mitochondria, yolk, and 

 pigment, one can observe a trace of granules appearing as a fine line 

 across the egg, somewhat below the oil cap, if examined with high 

 magnification (700 x) just after centrifuging'. This line separates the 

 clear area into two distinct portions. Sometimes small fibers are observ- 

 able among the granules (E. B. Harvey, 1946a). These two portions of 

 the clear layer stain with different intensity with many vital dyes (E. B. 

 Harvey, 1941c). They have been studied by McCuUoch (1952a), and 

 the fibers studied with the electron microscope. 



In order to get any development, it is necessary to allow the clear 

 quarters to stand for an hour or two before they are fertilized. Other- 

 wise the fertilization membrane? is not formed or is so thin that it rup- 

 tures. The stages following fertilization are exactly like those of a 

 normal egg except that the fertilization membrane isthin, theperivitel- 

 line space small, and the astral fibers are not visible because of lack 

 of granules. The whole process is slower. A normal cleavage may take 

 place in 2 hours instead of 50 minutes, which is the time for the whole 

 egg and white half. The cleavage plane may be in any relation to 

 the oil cap, so that one of the blastomeres contains all, none, or any 

 portion of the oil. The second cleavage, resulting in four equal cells, 

 usually takes place in about 3 hours instead of i| hours as in the nor- 

 mal egg. Successive cleavages follow, all delayed, till a normal blastula 

 is formed. The blastula hatches from the fertilization membrane and 

 becomes free-swimming in about 20 hours or longer (instead of 8 hours). 

 These blastulae are much less granular than those f.om white halves. 

 Some of these clear quarter blastulae have formed normal gastrulae 



