20 J. T. PATTEESON 



protoplasm toward the posterior end. This results in trans- 

 forming the elongated, pear-shaped egg into a figure more or 

 less circular in outhne. The number of polar nuclei at this 

 stage is sixteen. In one egg all sixteen polar nuclei are dividing 

 simultaneously (fig. 44) . After this period these divisions become 

 hregiilar, as indicated by the fact that one frequently finds single 

 nuclei in mitosis. 



/. The morula stage 



After the twenty-eight-celled stage all synchrony in division 

 is lost, and one may find from one to several blastomeres under- 

 going division in any egg. Consequently a typical fifty-six- or 

 112-celled stage is not seen. We may therefore consider together 

 several eggs which represent steps leading up to the formation 

 of a solid-ball stage, or what may be called the morula stage. 



Figure 45 is a section through a fiftj^-two-celled stage. There 

 are present four nucleolar cells, showing that these cells have not 

 further divided. Figure 46 represents a sixty-celled stage. The 

 polar membrane is remarkably clear and of almost equal thick- 

 ness aroimd the entire egg. This stage represents a condition 

 characteristic of this period of development, viz., a tendency 

 in certain cells for the cytoplasm to become drawn out into an 

 elongated process. Such cells often become spindle-shaped. 



Figure 47 is a median section through an egg composed of 

 about seventy cells. It is of peculiar interest in that it represents 

 the most advanced stage in which one can recognize the de- 

 scendants of the nucleolar blastomere. In the section drawn 

 five of these are clearly visible, and a sixth hes in an adjacent 

 section. It is clear that an irregularity in the division of these 

 cells has already set in, which is further evidenced in other eggs 

 showing but one of the four nucleolar cells undergoing division. 

 The nucleolar cells therefore follow the rule of loss of synchrony in 

 divisions as do the other cells. 



Figure 48 is a median section of a 135-celled stage, which has 

 to a remarkable degree retained the original pear-Hke shape of the 

 egg. At the anterior end there exists a rather interesting con- 

 dition, which has been noted in some other eggs. A single 



