186 



GENERAL EMBRYOLOGY 



evident, the position of the plane of symmetry of the zygote, 

 and hence of the embryo. In a few cases the nuclei are some- 

 what amoeboid, in others they seem to be carried by proto- 

 plasmic currents, and in still others they seem directed by the 



FIG. 96. Diagrams showing the paths of the germ-nuclei in four different eggs 

 of the sea-urchin Toxopneustes. From camera drawings of the transparent 

 living eggs. From Wilson, "Cell." In all the figures the original position of 

 the egg-nucleus (reticulated) is shown at 9 ; the point at which the spermatozoon 

 enters at E (entrance-cone). Arrows indicate the paths traversed by the nuclei. 

 At the meeting-point (M) the egg-nucleus is dotted. The cleavage-nucleus in 

 its final position is ruled in parallel lines, and through it is drawn the axis of the 

 resulting cleavage-figure. The axis of the egg is indicated by an arrow, the 

 point of which is turned away from the micromere-pole. Plane of first cleavage, 

 passing near the entrance-point, shown by the curved dotted line. 



location of the asters. These have grown to a considerable 

 size now, and seem mechanically forced into that position where 

 the tensions between the cytoplasm and the more or less rigid 

 asters reach an equilibrium. Most frequently this is the center 

 of the cytoplasmic mass, so that ultimately the two nuclei 



