MUSEUM OF COMPARATIVE ZOOLOGY. 251 
second furrow on each side, and especially at the peripheral extremities, 
or that part most distant from the primary plane of cleavage, exhibit 
rhizopodal elevations similar to those which accompany the formation 
of the primary furrow, and which we shall later see are found to form 
especially in later stages of growth, wherever a new plane is about to 
appear. Similar rhizopodal phenomena are also prerhonitory of death 
in the cells of the egg. 
At 1h. 30 m. P. m. (fig. 19), half an hour after the secondary cleavage 
furrow began to appear, the secondary groove (se.) shows signs of clos- 
ing, and the walls draw together to form the second cleavage plane 
(2cl.pl.). The closure of the secondary furrow takes place in substan- 
tially the same manner as the primary, and begins at the junction with 
the primary, working gradually to the periphery. All the time that 
the growing together of the sides of the furrow is going on, as the 
movement of closure advances towards the equator it is accompanied 
by the formation of new folds and the pushing out of pseudopodia in 
the line of its advance. In my figure representing the egg at 1h. 30m. 
p.M. these folds can be seen in the left hand of the figure, where the 
furrow is only partly closed. 
By the closure of the second furrow, combined with the contortion 
which is thus caused in the primary plane of cleavage, the profile of the 
first plane (primary), pr., appears zigzag, or the line which was formerly 
vertical is now not straight from one pole to the opposite, but is broken 
midway in its course. As this vertical marks the direction of the pri- 
mary cleavage plane (1 cl. p/.), we have indications that the primary 
cleavage plane, once intact, is now broken or bent. That modification 
in this plane can be recognized in later stages of development, being 
seen as late as the 8-cell stage. The diameter of the egg on the pri- 
mary cleavage-plane is about .60mm.; on a plane at right angles, 
45mm. The segmentation spheres have no visible nuclei. The great 
mass of the ovum is transparent, and the part surrounding the upper 
end of the vertical line, which is the primary plane of cleavage, is of a 
rosy color. 
The next stage of cleavage, 1h. 35m. p.m. (fig. 20), thirty-five min- 
utes after the beginning of the modification of the 2-cell stage, differs 
very slightly from that just described. The second cleavage furrow 
(2 cl. pl.) is now closed almost to its very periphery, although protoplas- 
mic elevations are seen at intervals along the furrow, a sure sign that 
the process is not yet completed. Remnants of the unclosed furrow 
are seen at each end of the horizontal furrow (2 cd. pi.). 
