737 
double cleavage with that of normal single embryos proves that in 
the former the mode of cleavage changes, so that the 
blastomeres of the half-sized 8-celled stage cannot 
individually be identified with those of one half the 
normal 16-celled stage. In the normal development the third 
cleavage is equatorial and the fourth more or less nearly meridional. 
In the true double development the reverse is true, the third (common) 
cleavage being meridional (or rather vertical) and the fourth (third 
cleavage of the twin half) equatorial. 
This conclusion, which is by no means as evident as it may at 
first sight appear, rests on the following facts. The first indication 
of duality is shown in forms of the bilateral type that are transver- 
sely elongated (fig. 2, D) but otherwise agree with the normal em- 
bryos. The genetic connections of the cells in such forms are the 
same as in the normal cleavage, i. e. (for the left embryo-half) 1—2; 
3—4; 5—6; 7—8. That is, the four micromeres of each half (3, 4, 
5, 6) arose by the fission of two parent micromeres in the 8-celled 
stage, as shown in fig. 2, B. In the true double cleavage, on the 
other hand, the genetic connections of the cells in the corresponding 
stage are as follows: 1—3; 2—4; 7—5; 8—6; i.e, the four micro- 
meres of each half are derived separately from the four respective 
macromeres, and not by the fission of two parent micromeres. That 
this is the true relation is proved by the existence of transitional 
forms in which the cells have the some arrangement as in true double 
embryos but show a mixed mode of origin. Such a form, exactly in- 
termediate between the normal and the double cleavage is shown in 
fig. 2, E. The nuclear spindles (all distinctly visible in the pre- 
paration) leave no doubt as to the origin of the cells, and demonstrate 
the mode of transition. In each half (we may consider only the left 
half, since the two are alike) four micromeres are in process of for- 
mation. Two of them (3—4) agree with the normal cleavage, arising 
by the vertical fission of a parent micromere of the (common) 8-celled 
stage (cf. fig. 2, B). The other two (5, 6) are in process of separate 
formation from the two corresponding macromeres (7, 8). Thus, the 
upper half of the figure follows the normal mode of cleavage, while the 
lower half shows the regular double cleavage. Inspection of the figure 
will show that the disturbance in the form of cleavage showed itself at the 
the third (common) division which instead of being as usual equatorial 
in all of the four blastomeres became vertical in two of them. In the true 
double cleavage the third division is vertical in all four. The following 
diagram gives a comparison of the genetic relationships in the three forms: 
48 * 
