INTERNAL FACTORS 



IV. i 



fertilization can take place from any meridian, and that the 

 point of entry so selected becomes the ventral side later on ; in 

 other words, the fertilization meridian becomes the sagittal plane. 

 This, as we have seen, is true, or approximately true. Roux, 

 however, believed that the plane of the first furrow also coincided 

 with the other two, in fact that it was the first furrow which 

 determined the sagittal plane, and he brought forward evidence 

 to show that the first furrow, if it did not pass through the 

 point of entry of the sperm, at least either included, or was 

 parallel to, the inner end of the crooked path of the spermatozoon 

 within the egg, the so-called ' copulation ' path (Fig. 150). As we 

 now know there is very little correlation between the first furrow 

 and the sagittal plane. Nevertheless Roux's work remains of 



FIG. 150. Roux's diagrams to show the relation of the sperm-path 

 (Pig.) to the first furrow in the Frog's egg. In A the furrow includes the 

 sperm-path, in B it is parallel to it, in c it is parallel to the inner portion 

 of the path (copulation path), in D it includes only the very last portion 

 of the copulation path. (From Korschelt and Heider, after Eoux.) 



the greatest significance, for it seems extremely likely that, 

 while it is the actual point of entrance of the sperm and the 

 first, radially directed, part ('penetration path') of its track 

 within the egg, which determines the position of the grey 

 crescent, and so of the sagittal plane, it is the second part which 

 determines the direction of the first furrow, since the centro- 

 some will divide at right angles to this ' copulation ' path, or 

 line of junction of the two pronuclei ; the axis of the fertilization 

 spindle is of course given by the line of separation of the two 

 centrosomes, its equator by the plane including the two pro- 

 nuclei and the egg-axis, and this spindle-equator is the plane of 

 the first furrow. 1 



In the egg of the sea-urchin Toxopneiistes the definitive egg- 

 axis would appear to be fixed not by the original position of the egg- 

 1 See, however, Appendix A. 



