KOFOID: DEVELOPMENT OF LIMAX. 79 
rotate. Here we have a complete histologieal differentiation, while as 
yet only a comparatively small number of cells are present. Whether 
or not there exists amy causal nexus between precocious development 
and the spiral type of cleavage, is a question upon which experimental 
embryology may be destined to throw some light ; as yet experimenta- 
tion has been confined to eggs having the radial or bilateral form of 
cleavage. 
The three forms of cleavage, radial, spiral, and bilateral, are undoubt- 
edly connected. Wilson (93, p. 600) has suggested that the spiral 
type is a modification of the radial, and owes its peculiarities to mechan- 
ical conditions, I would also suggest that spiral and bilateral types are 
very intimately connected. The spirally cleaving egg is essentially 
bilateral from the time that tho first cleavage plane appears, and an 
inspection of the tables of the cleavage of Nereis, Umbrella, and Limax 
shows that the embryo becomes predominantly bilateral as the spiral 
cleavage fades out. In Nereis the transition from the spiral to the 
bilateral period is abrupt; in Umbrella and Limax the two periods over- 
lap during several generations. The cleavages which succeed those of 
the spiral type are meridional and equatorial and I believe are to be 
referred to the bilateral rather than the radial type; indeed, in some 
cases, as in the division of 5.3 and 7.1 in Umbrella, the division ap- 
proaches very closely the typical bilateral cleavage of the tunicate ege, 
i.e. is symmetrical with reference to the median plane of the embryo. 
Wilson (92, p. 391) has referred the meridional cleavage of 5.3 to the 
radial type. In Nereis this quartet divides before the mesoderm 
appears; in Umbrella and Limax after it appears. When, however, in 
Nereis the quartet 7.15 divides equatorially after the mesoderm is 
formed, Wilson refers this division to the bilaterai type. .It seems 
to me that all these equatorial and meridional cleavages succeeding the 
spiral divisions both before and after the mesoblast appears must be 
referred to the bilateral period of the embryo and to the bilateral type 
of cleavage. 
The precise agreement of Umbrella, Nereis, and Limax in these first 
bilateral eleavages is evidently something more than mere accident. 
The meridional character of the division in two of the cases (Limax and 
Nereis) suggests the possibility of similar mechanical conditions. But 
if all the conditions in the two cases are compared more closely, it 
becomes clear that there are important differences, The cleavage in 
question takes place in Nereis at the twenty-nine-cell stage, in Limax 
at the forty-four, and a comparison of Figure 39 or Figure 41 (Plate 
