1889.] Segmentation of the Ovum, 467 
divide. Flemming maintains that the division is always 
lengthwiseof the V-shaped grain, but this has been contro- 
verted by Carney. How the division occurs in the mam- 
malian ovum is unknown. By division, however it is effected, 
the number of chromatine grains is doubled; they form two 
sets ; one set moves toward one pole, the other towards the 
other pole ; the grains of each set keep at the same level as 
they move, until they reach the end of the spindle, where they 
appear as a polar disc (Carnoy's couronne polaire). Next the 
achromatic threads of the spindle break through and are 
apparently drawn in towards each polar crown. There are 
now two nuclear masses, each near but not at the centre of a 
radiation, and each consisting of chromatine and achromatic 
substance, each mass develops into a complete membranate 
nucleus, but the steps of this process have yet to be followed 
in detail in the vertebrate ovum. 
The signs of division of the protoplasm usually become 
visible about the time the polar crowns are formed, but when 
the ovum contains much deutoplasm the division may be re- 
tarded. In the plane which passes through the equator of the 
nuclear spindle, there appears a furrow on the surface of the 
ovum, which gradually spreads and deepens until it is a com- 
plete fissure around the cell, it cuts in deeper until at last only 
a thin stalk connects the two halves of the cell, and thereupon 
the stalk breaks and the cell is divided. There next ensues a 
pause, during which the astral rays of the protoplasm disap- 
pears in the daughter cells, and the daughter nuclei assume 
each the form of an ordinary resting membranate nucleus. 
The external appearances of segmentation in the living 
ovum vary, of course, especially according to the amount and 
distribution of the yolk material. The appearances in holo- 
blastic ova with very little yolk are well exemplified by Limax 
campestris. Mark's description, 32, is, nearly in his own words, 
as follows : 
" In Limax, after impregnation, the region of the segmenta- 
tion nucleus remains more clear, but all that can be distin- 
guished is a more or less circular ill-defined area, which is less 
