KOFOID: DEVELOPMENT OF LIMAX. NS 
period in the seventh generation. The cleavages of the following gen- 
erations are meridional or equatorial, and belong to the bilateral period. 
Another noticeable feature is the general precedence of the mesoderm- 
producing quadrant d in the cleavages of the various quartets. 
The bilateral period in the cleavage of Umbrella is not so sharply 
marked off from the spiral period as it is in Nereis ; in Umbrella spiral 
cleavage occurs as late as the ninth generation. In Nereis it ceases in 
the seventh. The quartets 5.3 and 0.2 ¿n both forms are the ones in 
which the spiral character of the division first gives way to the merid- 
characteristic of the bilateral period. 
ional and equatorial cleavage, 
The cleavage of Umbrella, like that of Nereis, presents no contradic- 
tions to the law of alternation of spirals. "This striking agreement of 
Nercis, Umbrella, and Limax must far outweigh any seeming contra- 
dietion arising in the work of the earlier writers upon spiral cleavage. 
It is only necessary to apply the proposed system of nomenclature to the 
careful work of Wilson and Heymons to make clear at once that the 
alternation defended holds good. The system of nomenclature employed 
in this paper facilitates the demonstration of the alternation of spirals 
in successive generations of cells ; but the alternation itself is a factor 
independent of mere names. It is the fundamental basis of the so 
called “spiral type” of cleavage. A recognition of this fact might well 
be embodied in nomenclature, and alternating cleavage substituted for 
the ambiguous and misleading term * spiral cleavage." 
D. The Mesoderm. 
In the forty-four-cell stage, at which the discussion of the cleavage of 
Limax was dropped, the germ layers are already differentiated. The 
quartets 7.3, 7.4, 6.3, 7.7, 7.8, 6.5, 6.6, 6.7, 6.8, are pure ectoderm, the 
quartet 7.1 and three fourths of the quartet 7.2, viz. a, 6", and c™, are 
entoderm, while d^? is the sole representative of the middle germ layer. 
The seven entoderm cells and their progeny come to lie in the region 
of the blastopore, and with the invagination are carried in to form the 
lining of the archenteron. The primary mesoblast divides bilaterally, 
i.e. in the median plane of the embryo, shortly after the forty-four-cell 
stage. The two mesoblasts retain a slight connection with the exterior, 
and at the ninety-cell stage have each divided transversely, the periph- 
eral and posterior pair of cells are the smaller, and retain a slight con- 
nection with the exterior. The next division occurs in the anterior 
pair. The cells of the mesoderm continue to multiply until there are 
