EFFECT 'OF HEAT ON THE EGG OF CUMINGIA. 2JI 



somes are qualitatively different from each other. On the other 

 hand the facts are not incompatible with the opposing view that 

 the chromatin is a homogeneous substance, which condenses at 

 mitosis into masses devoid of individuality. 



SUMMARY. 



1. If the eggs of Cumingia are subjected to heat immediately 

 after fertilization, they do not form polar bodies. The chromo- 

 somes of the first polar spindle divide and two resting nuclei are 

 formed. These nuclei fuse with each other and with the male 

 pronucleus, giving rise to a cleavage nucleus which contains a 

 triploid amount of chromatin. 



2. The equatorial plates of these eggs show forty-five to sixty 

 chromosomes. About the same number has been found in the 

 cleavage of parthenogenetic (diploid) eggs; but there the indi- 

 vidual bodies are smaller than they are in the triploid eggs. 



3. If fertilized eggs are heated after they have formed polar 

 bodies, the equatorial plates of their first cleavage contain fifty- 

 five to sixty-five chromosomes. These are equal in size to those 

 of a triploid egg. 



4. Study of later stages (resting nuclei and equatorial plates) 

 shows that the chromatin of the polar nucleus is active, and 

 approximately equivalent to that of a male pronucleus. 



5. The size and number of the chromosomes in heated eggs is 

 not dependent on the amount of chromatin that is present. They 

 vary with the susceptibility of the egg to heat. 



6. The conditions described are not in opposition to the theory 

 of the individuality of the chromosomes, if that theory is given 

 a broad interpretation. 



BIBLIOGRAPHY. 

 Boveri, Th. 



'05 Zellen Studien, V. Jena, 1905. 

 Hance, Robert T. 



'i8a Somatic Chromosome Variations in the Evening Primrose. (Enothera 



scintillans. Genetics. 

 'i8b The Diploid Chromosome Complexes of the Pig (Sus scrofa) and Their 



Variations, Jour. Morph., Vol. 30, No. i. 

 Jordan, H. E. 



'10 A Cytological Study of the Egg of Cumingia with Special Reference to the 



History of the Chromosomes and the Centrosome, Arch. fur. Zellforsch., 



Vol. 4. 



