322 WILSON AND MATHEWS. [Vol. X. 



The paths of the pronuclei vary extremely according to the 

 point at which the sperm enters (Fig. i). The entrance-path 

 of the sperm is nearly, though never quite, radial, and the 

 copulation-path forms but a slight angle with it. That of the 

 egg-nucleus may be in any direction from its original position 

 (even outwards toward the periphery), and varies with the 

 entrance-point of the sperm. The meeting-point of the pronuclei 

 is without constant relation to the egg-axis, or the original posi- 

 tion of the egg-nucleus. It is seldom at the actual center of 

 the Q^g. After union of the pronuclei the segmentation-nucleus 

 gradually moves to a position near but 7iot at the center of the 

 ^gg, and in this eccentric position the first cleavage-amphiaster 

 is subsequently developed, the first cleavage-furrow invariably 

 appearing first on that side nearest the amphiaster, and extend- 

 ing thence around and through the Q.gg. The eccentricity 

 persists on the same side in the 2-celled and 4-celled stages, 

 and the later history of the o.^^ shows that the micromeres of 

 the l6-cell stage are formed at that pole of the egg opposite to 

 the eccentric segmentation-micleus} In other words, the eccen- 

 tricity of the cleavage-nucleus marks the definitive polarity of 

 the egg. 



In view of this fact it might have been expected that the 

 eccentricity of the cleavage-nucleus would be in the same 

 radius with that of the egg-nucleus. Such, however, is rarely 

 the case. It is a remarkable fact (illustrated by the diagrams 

 in Fig. I, which are selected from a large number of observed 

 cases), that the eccetitricity of the cleavage-nucleus has no con- 

 stant relation to that of the egg-nucleus, and may be in any 

 radius of the egg. In other words, tJie definitive egg-axis {polar 

 axis) may form any angle with the original axis passing through 

 the egg-nucleiis . This fact, accurately determined in a large 

 number of cases, leaves no escape from the following alterna- 

 tive : If we assume the polarity of the ^gg to be pre-determined 

 from the beginning, we must admit that the polarity determines 

 the position of the segmentation-nucleus, but is without influ- 

 ence on that of the egg-nucleus before fertilization, so that the 

 latter may wander to any position. If, on the other hand, the 



1 Cf. Morgan, Anat. Ahz., IX, 1894. 



