38 Hyhridization of Echinoids. 



nuclear mass occurred between the 64-cell stage and the blastula 

 stage, this would mean an increase of about 37 per cent with the 

 division of each cell. Godlewski speaks of the increase as being 

 nearly in a geometric ratio, this calculation being based on the volume 

 of nuclear mass in the unfertilized egg. It should be noted that 

 Erdmann's figures are of volume of chromosomes and that Godlew- 

 ski's are of nuclear volume. Conklin (1912) has estimated that in 

 Crepidula the chromosomal mass grows at the rate of 8 per cent for 

 each division up to the 32-cell stage. 



Baltzer (1909) has indicated the large probable error in estimates 

 of total chromatin volume that are based on the assumption that 

 the chromosomes in an Echinoid egg are of equal size. 



Mathews believes that nucleic acid is the chromatin of the his- 

 tologists. We have seen that JMasing found no evidence of increase 

 of nucleic acid during development in Arhacia up to the blastula 

 stage. The evidence of Conkhn, Erdmann, and Godlewski is con- 

 clusive in its demonstration of increase of total volume of chromo- 

 somes during this period. If there has been no increase in the amount 

 of nucleic acid, there must have been increase in its basic protein 

 portion. 



Goldschmidt's (1917) idea that the chromosomes act as adsorbents 

 of enzymes which constitute the chemical basis of heredity finds 

 much to support it in the facts demonstrated in this paper. We are 

 able to show the presence of nucleic acid by our stains; we have no 

 means of making the nuclear enzymes visible. Our only test for 

 them is in the things that they do. We can not hope to demonstrate 

 consecutive stages in processes of synthesis by specific stains. We 

 can demonstrate, and the material discussed in this paper has 

 demonstrated, the fact that foreign nuclear material produces reac- 

 tions in the cytoplasm that do not occur in straight-fertilized eggs. 



Closeness of relationship is by no means indicative of the readiness 

 with which the initial impulse to development may be received, nor 

 a sure criterion of the extent to which it may proceed. In the Cidaris- 

 Lytechinus and Cidaris-Tripneustes crosses under consideration there 

 seems to be little cortical block to the entrance of the spermatozoon. 

 There is little internal block during early development. Develop- 

 ment proceeds regularly to the period immediately before gastrula- 

 tion. To this point it has been following the general path of develop- 

 ment taken by most Echinoderms. At the point of deviation of 

 special from general, abnormalities appear. Development ends in 

 the gastrula stage, as in many crosses between Echinoderms. The 

 cells which should have gone to the completion of the archenteron 

 and the prospective mesenchyme cells are the first to die. Sections 

 show the nuclei to be swollen with chromatin. The nuclei finally 

 burst, extruding irregular masses of chromatin into the cytoplasm. 



