216 BUNTING. fiViorLIOKe 
When first laid, the egg is sometimes oval, although usually 
globular, having an average diameter of 0.16 mm, On being 
discharged, the ova sink at once to the bottom of the vessel in 
which they are contained. Experiments seem to show that 
the ova are fertilized either just previous to, or at the moment 
of their ejection. In about fifteen minutes after the ova are 
laid, the polar bodies (Pl. II, Fig. 22) appear. When first 
observed two globules were present, one had been extruded, 
while the other one was just appearing. One of the two divided 
subsequently, in a plane at right angles to the first cleavage 
plane of the ovum (PI. II, Fig. 23). Within ten minutes from 
the extrusion of the first polar body, the second was ejected. 
CLEAVAGE. 
The deutoplasm and protoplasm being evenly distributed in 
this ovum, there is no marked division into an animal and 
vegetative pole. 
The first cleavage takes place about forty minutes after the 
egg is laid. The plane of division is vertical, cutting in from 
the point where the polar bodies pass off, and dividing the egg 
into equal right and left halves. The first furrow, when 
viewed from above, appears as a cup-shaped depression, as is 
seen in Pl, II, Fig. 22. This progresses slowly, until at the 
end of ten minutes, the two blastomeres are simply connected 
by a slight protoplasmic thread (Pl. II, Fig. 24). In this 
thread protoplasmic movements are often seen. At the end 
of fifteen minutes, the complete two-celled stage is reached. 
The second cleavage planes start from the center about a 
half an hour after the first cleavage begins, and work their way 
gradually to the outside of the egg. During this cleavage 
there is frequently a movement of the blastomeres, which com- 
pletely changes their relative positions to each other. At the 
completion of this cleavage they again assume very nearly their 
former positions. At first they appear as four cells around a 
central cavity as is shown by Dr. E. B. Wilson in Diagram 
VIII, A, of his paper upon Nereis, for the four-celled stage of 
the true radial type, represented by Amphioxus and Synapta; 
