kofoid: development of limax. 79 



rotate. Here we have a complete histological differentiation, wliilc as 

 yet only a comparatively small number of cells are present. Wiiethcr 

 or not there exists any 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. GOO) 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 the 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 fiides 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 egg, 

 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 

 Nei-eis the quartet 7.15 divides equatorially after the mesoderm is 

 formed, Wilson refers this division to the bilateral 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 cleavages 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 siroilar mechanical conditions. But 

 if all the conditions in the two cases are compared more closely, it 

 becomes clear that there are important diff'erences. 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 



I 



