80 OUTLINES OF CHORDATE DEVELOPMENT 



in the usual way, putting the middle piece with its centrosome 

 in advance of the head, which begins to dissolve and to form 

 a typical vesicular and enlarged nucleus. Then the sperm 

 changes its course, often abruptly, and moves toward the region 

 where the male and female nuclei will unite, unless, indeed, 

 the penetration path may have led in that direction (Fig. 27, 



FIG. 27. Sections through the egg of R. fusca, showing penetration and copu- 

 lation paths, and the symmetry of the first cleavage plane. After O. Schultze. 



A. Sagittal section through the egg before the appearance of the first cleavage; 



B. Frontal section of the same stage as A, showing the symmetrical distribution 

 of the egg materials. C. Frontal section through egg in two-cell stage, showing 

 the symmetry of the egg; the penetration path is not shown, a, Anterior; 

 cp, copulation path; I, left; p, posterior; pp, penetration path; r, right; s, remains 

 of first cleavage spindle; sp, superficial pigment; 1, first cleavage furrow. 



A). This second part of the sperm path is known as the copu- 

 lation path and like the penetration path, it is marked by a 

 trail of pigment left in the cytoplasm. 



Swelling of the Egg Membranes. One result of the entrance 

 of the sperm is the withdrawal of fluid from the egg substance. 

 This fluid accumulates between the surface of the egg and the 

 chorion forming the perivitelline space. This leaves the egg free 



