FERTILIZATION IN PLATYNEREIS MEGALOPS 223 
The granules in the region may appear as a disc, but never as 
a retracted cone, as in Nereis. The cortical breakdown has 
released the close application of the yolk spheres to the inner 
cortical margin; they are now irregularly spaced and among 
them lies the granular cytoplasm. 
The figures (2 to 5) also give good pictures of the spermatozoa. 
They are much like the living spermatozoon. The head is 
almost spherical, the perforatorium a large blunt cap; the mid- 
dle-piece and tail are often clearly defined. 
2. Penetration of the spermatozoon. The penetration of the 
sperm head begins at twenty to twenty-six minutes after laying 
(cf. Nereis, forty-five minutes after insemination). The first 
maturation spindle, in the metaphase, is oriented in the polar 
plane of the egg; the inner endoplasmic mass which incloses 
the spindle is, at this stage, triangular in section; the outer aster 
of the spindle is near the apex of the triangle. The base of the 
triangle is less blunt than in previous stages and reaches farther 
outward along a radius of the egg. The various stages of pene- 
tration are shown in figures 6 to 18. The sperm substance enters 
the egg as a slender black thread, which gradually increases in 
size at its inner end. The sperm head, in my preparations, is 
usually homogeneously black, but often the external bulb is 
not so dark; or lighter areas appear along the entering thread; 
(particularly figs. 10,11, 12, and 14). Often, especially in sec- 
tions stained for twelve hours only, in stages just after the attach- 
ment of the perforatorium to the cytoplasm, the head appears, 
not as a homogeneous chromatin mass, but as a slightly differ- 
entiated body. One gains, therefore, the impression that the 
spermatozoon flows into the egg (cf. Koltzoff and Lillie, who, 
with different methods, find Nereid spermatozoa extremely 
ductile). 
Cytoplasmic changes due to sperm entry are clearly marked 
during the later stages of penetration; striae appear in the cyto- 
plasm around the entering spermatozoon, the area stains more 
deeply, and a projection from the endoplasmic mass reaches out 
toward the point of sperm entry (see figs. 14 to 17) (cf. on these 
points, Foot, Gardiner, Vedjovsky, Jenkinson, and Lillie, ’12). 
