470 CHARLES RUSSELL BARDEEN 



at different stages of development. Fertilized eggs of the frog 

 kept at a temperature abov.e a maximum which varies for differ- 

 ent species, develop abnormally. At the animal pole cleavage is 

 very rapid; at the vegetable it is very slow and spina bifida larvae 

 are frequently produced. Freshly fertilized eggs placed in water 

 at a temperature of 0° Centigrade and kept there twenty-four 

 hours develop abnormally, (Hertwig) while in the gastrulation 

 stages egg may be kept at a low temperature for fourteen days and 

 still develop normally when brought to the room temperature. 

 (Schultze) 



E. Godlewsky (1908), found in sea urchin eggs no increase in 

 the amount of nuclear material in the first two blastomeres. 

 Each nucleus was approximately half the size of the nucleus of 

 the fertilized ovum. The proportion of nuclear to cytoplasmic 

 material was estimated as 550:1 in the unfertilized egg; as 275:1 

 in each of the first two blastomeres. During cleavage from the 

 two to the sixty-four cell stages there is a very rapid production 

 of chromatin. The nuclei increase in number but remain about 

 the same size. The proportion of nuclear cytoplasmic material 

 increases from 275:1 to 12:1. From the sixty-four cell stage to 

 the termination of the blastula stage the nuclei increase rapidly 

 in number but decrease so much in size that the total amount of 

 nuclear material is relatively slightly increased. The proportion 

 of nuclear to cytoplasmic material changes from 12:1 to 6:1. 

 With the decrease in size of the nuclei a relatively greater amount 

 of nuclear surface is presented to the cytoplasm. During the 

 gastrula and pluteus stages as the cells multiply there is a gradual 

 increase in the amount of chromatin in the organism. The nuclei 

 do not change much in size. 



Similar studies on amphibian eggs have not, so far as I am aware, 

 been made. It is highly probable, however, that the relations of 

 nuclear to cytoplasmic material during cleavage are homologous. 

 The period of greatest susceptibility of amphibian ova to the X 

 rays, therefore, probably corresponds with the period of greatest 

 relative activity in the production of nuclear at the expense of 

 cytoplasmic material. If this be true we should expect that in 



