BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
B. Discussion of Cleavage. 
SECOND GENERATION. First CLEAVAGE PLANE, Two CELLS. 
Plate I. Fig. 14. 
It should be noted that the phrase “generation of cells ” is used in its 
strict genealogical meaning, and not in the sense in which Fol (^75, ”76), 
Blochmann (’81), McMurrich (86), and Heymons (93) have used the 
phrases “generation of micromeres ” and “ generation of cells.” The pro- 
cesses of maturation, fecundation, and the formation of the first cleavage 
furrow have been described in detail in Dr. Mark's classic work upon 
Limax campestris. His published work was not carried beyond this 
stage, though he had continued his researches far into the later stages 
of development. The appearance of Fols work (80), which dealt 
largely with the embryology of Limax, and the pressure of other duties, 
have interfered with the completion and publication of his studies. Dr. 
Mark has very kindly placed his numerous drawings and careful notes in 
my hands, and they have been of invaluable assistance to me in my work. 
I shall not attempt to add to his complete description of the formation 
of the first cleavage plane, but shall begin my work with the stage 
represented in Plate I. Fig. 14. This is a lateral view of an egg which 
has just completed the first division. Warneck (50) has stated that 
in Limax and Lymneus this plane is oblique to the axis of elongation, 
instead of transverse to it, and has distinctly said that this conclusion 
was not based upon a deceptive orientation of tho egg. Fol (75) has 
described a similar occurrence in Cymbulia. I have found no evidence 
that in the least confirms this statement of Warneck’s. At the stage 
shown in Figure 14 the two nuclei lie close to the approximated sur- 
faces of the blastomeres, at a level about midway between the animal and 
vegetative poles ; they are still quite small, and have only a very deli- 
cate membrane. Fach has an elongated oval outline, with the long axis 
extending toward the astrocwl of the cell in which it lies. Their 
position indicates that in the progress of the cleavage furrow toward the 
vegetative pole the nuclei (daughter segments) were in some way car- 
ried downward toward that pole. Mark (81) has described such a pro- 
cedure in the eggs of Limax campestris. There are a number of decply 
staining granulations in the peripheral part of the cell adjacent to the 
nuclei, which probably constitute the remnant of the cell plate ; there 
is thus every indication of recent coll division. 
The astrocoels appear as clear areas, almost as large as the nuclei, 
containing a few scattered deeply staining granules. ‘These clear arcas 
