MATURATION AND FECUNDATION OF THE OYUM. 123 
brane when the figure had reached the surface at the opposite 
pole. 
The two poles of the similar branches both reach the 
surface of the vitellus. The angle between these branches 
now becomes more and more obtuse, until the prothyalosoma 
reaches the surface of the ovum. 
The groups of achromatic granules at the poles of the similar 
branches become, by the fusion of the granules, homogeneous 
discs. A similar fusion takes place among both the axial and 
peripheral achromatic fibrils of the figure ; at the same time 
the whole figure becomes smaller. By this time the sperma- 
tozoon has passed to the centre of the ovum, and fibrils are 
seen going from it to reach by a curved course the vertical 
branch of the Y. The vitellus has now lost all appearance of 
formed elements except its protoplasmic reticulum, and is very 
transparent ; but after this commences a formation of granules 
round the spermatozoon, which gradually extends throughout 
the vitellus till the whole is granular and somewhat opaque, and 
the protoplasmic reticulum is obscured. 
The next change in the ypsiliform figure is that its vertical 
branch becomes superficial, still remaining at right angles to 
the line connecting the poles of the similar branches; and then 
a new branch is formed opposite to the original vertical 
branch, so that a cross is formed at the surface of the vitellus 
with the prothyalosoma in its centre. The two last-formed 
branches of the cross undergo a longitudinal cleavage which 
seems to play an important part in the liberation of the polar 
globule. 
After this all the branches of the cross disappear, and 
nothing remains but a finely granular disc where the cross 
existed, surrounded by the granular vitellus. The periphery of 
the disc is distinctly defined, and in its centre are the prothya- 
losoma and its contained chromatic elements unchanged. 
Later even the limits of the disc become obscure. The prot- 
hyalosoma is somewhat reduced in size. 
The polar globule is formed by division of the prothyalosoma 
and its contained globules in a plane tangential to the surface 
