The Cytoplasm of the Sea Urchin Egg 



149 



nuclear membrane on both sides or were situated 

 within outpouchings of the nuclear membrane. 



The RNA-containing clear cytoplasm contained 

 so many different structures that it was impossible 

 to ascertain exactly what structure or structures were 

 responsible for the basophilic staining. There seemed 

 to be no particular structure that was confined to 

 this layer upon centrifugation. Many cell compo- 

 nents, however, were concentrated to this zone, 

 among them the small dense granules with an approxi- 

 mate diameter of 150 A (which normally were more 

 or less uniformly dispersed throughout the cyto- 

 plasm of the egg), and vesicles and membranous 

 systems, some of which were dotted with small gra- 

 nules on their outer surfaces. 



Discussion. — Quantitative biochemistry has pro- 

 vided data concerning the nucleic acid content of 

 the sea urchin eggs. Perhaps the most remarkable 

 result from such studies is the demonstration that 

 the DNA-content in the egg is approximately 25 

 times higher than that of the spermatozoon (8, 13). 

 This is interpreted to mean that there may exist a 

 cytoplasmic DNA-store in the egg cell (13, 21; see, 

 however, 8). The RNA content of the egg is approxi- 

 mately 30 times as great as that of the DNA (3, 18). 



Cytochemical localization of the two nucleic acids 

 has been performed on living eggs by ultra-violet 

 absorption at the wavelength 2537 A by Harvey 

 and Lavin (11, 12). There is good agreement between 

 these absorption pictures and pictures obtained by 

 cytochemical staining for nucleic acids. The centri- 

 fugal pole could not be investigated by the method of 

 Harvey and Lavin because the Arliacia egg contains 

 pigment that moves to the centrifugal pole and ab- 

 sorbs ultra-violet light as well as visible and infra- 

 red. The nucleus of the mature egg is Feulgen-nega- 

 tive (see (15), and for a differing opinion (6)), and 

 showed no detectable absorption in ultra-violet 

 light. 



In the present investigation the RNA has been 

 localized not only in the clear zone of the stratified 

 egg but also in particles in the centrifugal pole. 

 There are some reasons indicating that these heavy 

 particles are derived from the nucleus. They are 

 occasionally seen within bleb-like outpouchings of 

 the nuclear membrane, and in the cytoplasm they 

 are surrounded by a membrane that very closely 

 resembles the nuclear membrane. Like the nucleolus 

 they exhibit very great density on centrifugation. It 

 is suggested that either fragments from the nucleolus 

 or else other RNA-containing entities within the 

 nucleus are expelled through the nuclear membrane 

 together with portions of the membrane that act 

 as envelopes, it has been shown in an earlier paper 

 that fragments of the nuclear membrane may exist 



free in the egg cytoplasm (1); they are then often 

 arranged in groups of three to ten membranes 

 approximately parallel to each other. 



Three important basophilic entities have been 

 recognized in the cytoplasm of the sea urchin egg; 

 heavy bodies, "yolk nuclei", and the RNA-contai- 

 ning component of the clear cytoplasmic layer of 

 the stralilied egg. It is a notable feature that all 

 three entities contain a large number of granules 

 about 150 A in diameter. There is thus nothing that 

 speaks against the assumption that the 150 A granu- 

 les are responsible for the basophilic staining. Pa- 

 lade (16) demonstrated a good correlation between 

 cytoplasmic basophilia and "a small particulate 

 component of the cytoplasm" consisting of dense 

 granules of a diameter ranging between 80 and 300 A 

 (normally 100 to 150 A). In the sea urchin egg most 

 of the dense particles appear free in the cytoplasm, 

 and only in the "yolk nuclei" are they arranged on 

 membranes in a way characteristic of many verte- 

 brate tissue cells, to form a-cytomembranes accor- 

 ding to the terminology of Sjostrand (19). 



The author wishes to thank Dr. G. Gustafson, Kristi- 

 neberg Zoological Station, and Bestyrer D. Rustad, 

 Trondheinis Biological Station, for help and supply of 

 material; Dr. F. Sjostrand, Associate Professor of the 

 Department of Anatomy, for placing the resources of 

 his laboratory at the author's disposal; and Professor 

 J. Runnstrom, the Wenner-Gren Institute, for his unfail- 

 ing support and interest. Financial support from the 

 Swedish Natural Science Research Council is also grate- 

 fully acknowledged. 



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