52 Papers from the Marine Biological Laboratory at Tortugas. 



stance is vacuolated and frequently it is very finely alveolar or apparently 

 homogeneous (figs. 38, 48). Sometimes the chromatin condenses in the 

 center of this plastin mass, leaving numerous vacuoles behind and subse- 

 quently fragments leave the main mass. Sometimes the chromatin breaks up 

 into numerous small spherical bodies scattered over the plastin. Again, small 

 masses of chromatin leave the nucleolus one after another, each leaving" a 

 vacuole behind (here the chromatin was partly held as viscid drops in the 

 form of spherules in the plastin) until all the chromatin is extracted from 

 the plastin. All of the above processes of nucleolar dissolution are appar- 

 ently normal. There appears to be absolutely no uniformity in the manner 

 in which the nucleolus dissolves. The important thing seems to be that the 

 nucleolus should break up at maturation and be prepared for appropria- 

 tion and assimilation by different parts of the egg, the manner of its dissolu- 

 tion having no significance. In all cases a plastin remnant is left behind by 

 the chromatin, which appears to have no further function, but is straightway 

 resorbed by the cytoplasm (figs. 52, 54). The chromatin, on the other hand, 

 is distributed partly to the chromosomes, partly to the nuclear reticulum, 

 and occasionally persists in part as a " metanucleus." 



THE ORIGIN OF THE CHROMOSOMES. 



Concerning the origin of the chromosomes of the first maturation spindle, 

 varying opinions are held by different investigators, even regarding very 

 closely allied animal species. Some maintain that the chromosomes arise 

 exclusively from the nucleolus ; others hold that the nucleolus contributes 

 nothing to their formation ; and a third class holds that they arise in part from 

 the nucleolus and in part from the nuclear reticulum. A brief survey of 

 recent expressions of opinion on this point is desirable here, particularly 

 because I believe that my results indicate that the divergence of opinion is 

 not as great as at first appears, and that a nucleolar origin of chromosomes 

 does not really involve the question of the individuality of the chromosomes, 

 as some investigators have recently held in several cases where the chromo- 

 somes seemed to arise from the nucleolus. 



Among botanists a similar difference of opinion has arisen. Wager, 

 in his recent paper on the nucleolus in the root-tip cells of Phaseolus, gives 

 an excellent review of the literature on the nucleolus in the plant-cell. He 

 concludes that the nucleolus in many of the higher plants is really a portion 

 of the nuclear network and that it contributes some material at least to the 

 formation of the chromosomes. 



Manifestly where the nucleolus is a simple plastin body the question as 

 to its relation to the chromosomes does not arise. In amphibia, where many 

 chromatin nucleoli are present, this point has been much discussed. O. 

 Schultze (1887) states that in the ova of Rana and Triton the nucleolar 

 substance takes part in the formation of the chromosomes. Born (1894) is 



