DEVELOPMENT OF CENTRIFUGED EGGS AND FRACTIONS I45 



plutei, but no normal plutei have been obtained (Plate XII, Photo- 

 graph 16). They usually remain "Dauerblastulae." Often the first two 

 white blastomeres develop independently forming twins; this is doubt- 

 less due to the lack of the hyaline layer which is thrown off by centri- 

 fugal force as a ring or crescent. This can be readily seen in the perivi- 

 telline space if the fertilization membrane is ruptured (Plate XVI, 

 Photograph 6). The red half does not develop. This is in marked con- 

 trast to the red half obtained from unfertilized eggs which may cleave 

 quite normally after parthenogenetic treatment, though having no 

 nucleus or nuclear material. The red half from the fertilized egg has 

 nevertheless been in contact with nuclear material. Parthenogenetic 

 agents do not have any effect on this red half. It is only if the fertilized 

 egg is broken apart before the (^ and $ nuclei have united, that the red 

 half may divide, owing to the presence of the ^ nucleus (E. B. Harvey, 

 1933 b, 1940 b). 



k. Conclusions 



An artificial distribution of granules by centrifugal force does not radi- 

 cally change the course of development after the egg is fertilized. No 

 special granules are necessary for cleavage and development (oil, yolk, 

 pigment, and mitochondria), since any fraction may develop without 

 one or more of these, and the clear quarter, which lacks all of them 

 (except oil), may develop into a normal pluteus. As for the nuclei, it 

 is apparent that the male nucleus is not necessary for cleavage and 

 development, since parthenogenetic development takes place in many 

 forms, and, in Arbacia results in a perfect pluteus in both the whole 

 eggs and the white halves. The female nucleus is not necessary, since 

 merogonic development of fertilized non-nucleate fragments takes 

 place in many forms and may result in a perfect pluteus in the red 

 halves o{ Arbacia. 



It has been shown that cleavage and early development may take 

 place, in the parthenogenetic merogones, without any nucleus at all. 

 The essential material for cleavage and early development, therefore, 

 must be the matrix or clear material which is present in all the frac- 

 tions. In the living state, this is optically empty, except for the thin 

 line of granules segregated out under high centrifugal forces. When 

 fixed and stained with haematoxylin, the matrix appears filled with 

 very small granules, microsomes (Lyon, 1907, E. B. Harvey, 1940c, 

 Figure 124). It contains nucleoproteins, as shown by ultraviolet of a 

 wave length of 2537 A (E. B. Harvey and Lavin, 1944). It contains the 



