MUSEUM OF COMPARATIVE ZOOLOGY. 505 



plasm (" nucleoplasm ") to be, not yolk plasm, but the pronucleus 

 itself, and the bodies embraced in it to be pronucleoli, and by means of 

 this interpretation (which is of course extended to the subsequent gen- 

 erations of nuclei and their nucleoli) is enabled to avoid the contradic- 

 tory position in which Biitschli and others find themselves w^hen they 

 endeavor to explain the existence of several nuclei in a cell without 

 thereby interfering with the essential character of the cell as a uninu- 

 clear structure. 



It is to be remarked in the beginning, that Whitman saw only traces 

 of a nuclear plate in the archiamphiasters, and these were so uncertain 

 in character that they were entirely omitted from his drawings. This, 

 I think, will serve to explain why he has been less successful in tra- 

 cing the origin of the female pronucleus than some other observers 

 whose more favorable objects have proved very instructive on this 

 point. 



Immediately after the appearance of the second polar globule a circu- 

 lar space directly below the latter, which appears in fresh eggs as a pel- 

 lucid spot, is found in eggs treated with osmic acid and carmine to be 

 filled with a very fine granular substance which has the lead-gray tinge 

 and the feeble staining capacity characteristic of the germinal vesicle 

 when similarly treated. This is the remnant of the archiamphiaster, 

 and is called the female pronucleus. It is without membrane, perfectly 

 homogeneous, and forms the centre of a radial system. On the inner 

 (or deeper) side of the female pronucleus there are subsequently to be 

 seen two small highly refractive corpuscles in close apposition, and to- 

 gether lOju, in diameter. These are sharply defined, homogeneous, and 

 more deeply colored than the nucleoplasm. They are female pronucleoli. 

 Whitman gives no positive information as to the origin of these pro- 

 nucleoli, whether they have a genetic connection with definite parts of 

 the amphiaster or arise as new structures within the nucleoplasm. The 

 male nucleus, which comes into view" near the centre about this time, 

 presents the same appearance, with, however, only one pronucleolus. 

 Both pronuclei are surrounded with radial lines, and their longest axes 

 lie in the main axis of the egg. In later stages only one nucleus is 

 found, with its main axis oblique to that of the egg, but embracing 

 both the male and female pronucleoli. The radial lines are fainter. 

 Then this primary cleavage nucleus comes to lie a little eccentrically 

 towards the oral pole, and its axis is at right angles to that of the egg. 

 Its substance (nucleoplasm) is more strongly colored near the pronu- 

 cleoli than at the periphery. Later the pronucleoli have become larger, 



