Hybridization of Echidoids. 37 



DISCUSSION. 



Masing (1910), selecting for study one of the nuclear constit- 

 uents, nucleic acid, found that while during development in Arbacia 

 pnstulosa up to the blastula stage there is an increase of about a 

 thousand fold in nuclear mass, there is no perceptible increase in 

 the nucleic-acid content of the egg. He reached the conclusions that 

 the nucleic acid of the cleavage nuclei arises from a preformed stock 

 in the egg plasma and that nucleic acid and the chromatin of the his- 

 tologists must be different things. (Quoted from Godlewski, 1911.) 



During the period studied by Masing there has been no increase 

 in the mass of organism. Godlewski (1908) showed that the total 

 volume of plasma in the blastula is about one-third less than in the 

 unsegmented egg. The nuclear material has increased in volume 

 at the expense of the cytoplasm. Godlewski's investigations show a 

 change in ratio between cytoplasm and nucleus from 550:1 in the 

 unfertilized egg to 6:1 in the blastula. In the light of the results 

 of Conklin (1912) on the growth of protoplasm during cleavage, and 

 in the light of my own results from a study of the eggs of Arbacia 

 punctulata, as yet unpublished, and of those of Hibbard on the eggs 

 of Echinarachnius parma, in press, these figures need some revision. 

 This revision will not affect Godlewski's general conclusions. The 

 unpublished results mentioned above show that in the eggs of Arbacia 

 punciidata and Echinarachnius parma there is a considerable mass of 

 deutoplasmic material, which is in process of transformation to 

 cytoplasm during the cleavage stages. In the echinoderm egg, 

 despite its frequent transparent character, there is a stock of yolk 

 and fat. Cell volume in these eggs is not equivalent to cytoplasmic 

 volume. The growth in nuclear mass is due, in part at least, to the 

 assimilation of deutoplasm. 



Marcus (1906) and Erdmann (1908) have shown that the size 

 of the chromosomes diminishes during cleavage. Erdmann has 

 concluded that the chromosomes of the pluteus of Sirongylocentrotus 

 have only about one-fortieth the volume of those of the first cleavage 

 spindle. She has also determined that there is a constant increase in 

 total quantity of chromatin. Her figures show that at the stage of 

 the blastula without mesenchyme, which Godlewski has estimated 

 as a 1,256-cell stage, the chromosomal volume is 78 times greater than 

 it was at the time of the first division. This would amount to an 

 increase of about 6 per cent with the division of each cell. Godlewski 

 (1908) estimated that the total volume of nuclear material is 47 times 

 greater in the 1,256-cell stage than in the unfertilized egg, which 

 would be 23.5 times the total volume in the fertilized egg. Inasmuch 

 as he believed that practically all of this growth took place between 

 the 1-cell stage and the 04-cell stage, and that but little increase of 



