DEVELOPMENT OF CUCUMARIA ECHINATA 191 
lock, i.e. a pair situated diagonally do not lie parallel to each 
other but their ends approach af one pole, while the other pair 
approach at the other pole. In an extreme case, these two 
pairs come to lie in different planes, one pair being high above 
the other. 
Hight-Cell stage.—In consequence of the interlocking 
of the blastomeres in the preceding stage, the two tiers of blasto- 
meres in the eight-cell stage tend to lie shifting 45° above the 
other. Much irregularity in regard to the size and position of 
blastomeres is often met with. 
Sixteen-Cell stage.—Here eight blastomeres in a tier 
lie above the other set consisting of eight. Very frequently, 
however, each tier shows a zigzag arrangement, thus the alter- 
nating four are slightly above the remaining four, and in an 
extreme case they form a tier of themselves at each pole. 
Thirty-two-Cell stage.—Now the embryo is globular 
in shape jeaving a spacious blastocoele inside it. In the most 
regular cases there are four cells in a tier on each pole, and 
between these there are three tiers of eight cells arranged in 
zigzag rows. No remarkable difference in size among the 
blastomeres can usually be discerned. 
Above Sixty-four-Cell stage.—Now it is hardly 
possible to recognize any special arrangement of blastomeres. 
Hereafter those at and near the vegetative pole are found to 
be a little larger than those of the opposite pole. No coagulable 
matter is as yet found in the blastocoele. The blastula is found 
still wrapped within the egg-membrane. 
According to Kowalewsky (17),Selenka (45), Edwards 
(12),and Des Arts (2), the cleavage of the eggs of C. kirchs- 
bergii, C.planci,C. frondosa, Holothuria tubu- 
losa,and H. floridana is total and equal or approximately 
equal. Selenka noticedthat in C. planci the difference of 
size among blastomeres is evident only after the thirty-two- 
cell stage, and that the cleavage ends at the beginning of the 
second day. Edwards stated that in H. floridana the 
four-cell stage is reached within three hours and the sixteen- 
cell stage within four hours. Des Arts observed in the egg 
