KOFOID: DEVELOPMENT OF LIMAX. 43 
will be encountered in the application of the system. After the ninth 
generation of cells the exponents become exceedingly cumbersome, but 
this is an objection which applies to all other systems as much as, or to 
even a greater extent than, to this; and it may perhaps in many cases be 
avoided in the later stages by the introduction, for teloblasts and their 
progeny, of subordinate dichotomous systems based on combinations of the 
numerals 1 and 2, as introduced by Chabry (’87), and later adopted by 
Wilson (92) in his subordinate systems. It is also true that the system, 
as proposed, does not optically differentiate the macromeres and the 
primary, secondary, and tertiary micromeres where it is desirable to 
distinguish these groups or their immediate descendants. This however 
is readily accomplished by the use of differential type, or even by other 
letters of the alphabet than a, b, c, d, but used in the same order. 
There seems to be no doubt that this system can be applied wher- 
ever it is possible to divide the cleaving egg into equivalent quadrants, 
and thus to distinguish quartets of cells. I have myself applied it to 
the spiral cleavage described for various forms (see review of tho litera- 
ture), and my friend, Mr. Castle, has applied it suce cessfully to the 
bilateral cleavage of Ciona and to that of Clavelina as described by Van 
Beneden et Julin (’84). 
To make this system available in all cases, it is only necessary to 
apply the second exponent in a constant manner with reference to some 
spatial relations; e. e. in the case of Tunicate ole: avage, with reference 
to the sagittal and transverse planes, starting in all cases from the 
vegetative pole. 
In conclusion, it hardly needs to be suggested that the generation 
basis of comparison is about the only ono that can be employed between 
the various types of cleavage; and I would add that it promises to be 
useful in the diseussion of precocious development. 
IV. GENERAL SKETCH OF THE DEVELOPMENT. 
Limax has spiral cleavage of the typical form, the spirals alternating 
ight spirals resulting in the even genera- 
© 
in successive cell generations, r 
tions and left spirals in tho odd. The mesoderm is derived from the left 
posterior quadrant, and, as in Nereis, Umbrella, Crepidula, and Unio, 
the first mesoblast cell is 7? An ephemeral, recurrent cleavage cavity 
ge, í 
appears at the two-oell stage, and recurs as late as the completion of the 
period of gastrulation. This cavity is excrotory in function, and is 
