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 ho 

 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 the litera- 

 ture), and my friend, Mr. Castle, has applied it successfully 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 necessai'y to 

 apply the second exponent in a constant manner with reference to some 

 spatial relations ; e. g. in the case of Tunicate cleavage, 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 one that can be employed between 

 the various types of cleavage ; and I would add that it promises to be 

 useful in the discussion of precocious development. 



IV. GENERAL SKETCH OF THE DEVELOPMENT. 



Limax has spiral cleavage of the typical form, the spirals alternating 

 in successive cell generations, right spirals resulting in the even genera- 

 tions and left spirals in the odd. The mesoderm is derived from the left 

 posterior quadrant, and, as in Nereis, Umbrella, Crepidula, and Unio, 

 the first mesoblast cell is d 12 . An ephemeral, recurrent cleavage cavity 

 appears at the two-cell stage, and recurs as late as the completion of the 

 period of gnstrulation. This cavity is excretory in function, and is 



