140 THE AMERICAN ARBACIA 



characteristic are the permanent blastulae with imperfect skeleton, the 

 "Dauerblastulae". There seems to be no constant percentage of normal 

 development in any one batch of eggs, but certain whole batches 

 develop better than others. Pigment granules characteristic of 

 normal plutei appear in the white plutei after a few days. 



White half-eggs which are fertilized while still elongate after centri- 

 fuging develop like the elongate whole egg. The first cleavage plane 

 goes across the short axis and divides the egg unequally. By subsequent 

 cleavages, a white slipper-shaped blastula is formed. 



White half-eggs will also develop parthenogenetically if treated with 

 concentrated sea water, and develop into white plutei. 



d. Red Half-Eggs, Fertilized Merogones. Plate X 



The red half-egg has no nucleus at the time of fertilization, but after 

 fertilization the sperm nucleus with its accompanying aster may often 

 be seen 15-20 minutes after the fertilization membrane has been given 

 off. The fertilization membrane and the hyaline layer are much thicker 

 than in the white half-eggs. The sperm nucleus is at first very small, 

 but enlarges and is quite noticeable about 30 minutes after fertilization. 

 The radiations disappear, the nucleus enlarges and breaks down about 

 an hour after fertilization. A dumbbell-shaped nucleus or a very small 

 amphiaster can sometimes be seen in the living egg, and then two 

 nuclei (about 80 minutes after fertilization). In stained preparations 

 one sees a very narrow spindle in metaphase and anaphase with chro- 

 mosomes, and coarse astral fibers (Plate XVI, Photographs 12, 15). A 

 two-celled stage may occur, especially in eggs fertilized while elongate, 

 but often cell boundaries fail to come in. The first cleavage plane in the 

 elongate egg comes across the short axis of the egg. Sometimes fairly 

 regular cleavages result in a blastula of many cells with a small 

 blastocoel, but often the blastula is multinucleate without cell bound- 

 aries. Hatching has been observed 1 1 hours after fertilization, and 

 they become active swimmers. Several perfectly normal plutei with 

 lattice-like skeletons have been raised; they were very heavily pig- 

 mented, and of course much smaller than those from whole eggs (see 

 E. B. Harvey, 1932, 1940c, 1951). (Plate X, Photograph 16). 



e. Red Half-Eggs, Unfertilized. Parthenogenetic Merogones. 



Plate XI 



The red half-eggs, though having no nucleus, can be activated by a 

 parthenogenetic agent, hypertonic sea water or distilled water. They 



