Effect of Carbon Dioxide on Eggs of Ascaris. 63 



were normal, the remainder being either masses of disorganized 

 cells, or embryos in which the posterior end was differentiated, 

 together with the primordial germ cells. The problem was to 

 determine the causes of the abnormalities. Eggs were preserved 

 in all stages, stained and mounted in toto. 



Two distinct and independent causes were found for the ab- 

 normal development. 



The first of these was the fusion of the chromosomes in the 

 equatorial plate phase of the dividing Si-blastomere. (This is 

 the " Ur-somatic " cell in which the diminution process takes 

 place.) The fusion resulted in one of two things: (a) When the 

 fusion involved the greater part of all the chromosomes, the blas- 

 tomere did not divide. At the next division cycle, a tetraster 

 appeared with eight chromosomes (or their equivalents in small 

 "diminished" chromosomes) lying in the spindles. The tetraster 

 divided very irregularly and the result was the total disorganization 

 of the cells of the ectoderm. Such eggs gave rise to embryos which 

 failed to invaginate. (b) When the fusion involved the ends of the 

 chromosomes only, then division took place, but the chromatin 

 was unequally distributed to the two daughter blastomeres, A and 

 B. This led to an upsetting of the cleavage rhythm of these two 

 cells, the blastomere with less chromatin dividing more rapidly 

 than its mate. The Pi blastomere (the cell which gives rise to the 

 cells of the entoderm, mesoderm, the stomadeum cells, and the 

 primordial germ cells) and its derivatives divide normally through- 

 out development. It should be added that the Pi cell plays a 

 dominant role in the formation of the posterior end of the embryo. 

 When the division of the chromatin was very unequal, the ecto- 

 derm cells became so scattered that they did not take up their 

 proper places and a mass of disorganized cells resulted. When the 

 division was more nearly equal, it seems probable that partially 

 normal embryos resulted. 



The second type of abnormality consisted in a shifting of the 

 positions of the A and B blastomeres and their derivatives. Due 

 to this, the cells from the Pi blastomere which come into relation 

 to certain of the ectoderm cells to form the stomadeum do not find 

 their proper places. The posterior end of the embryo, coming, for 

 the most part, from the derivatives of the P x cell was normally 

 differentiated. 



