76 bulletin: museum of comparative zoology. 



The systom devised by Kofoid ('94) — which Castle applied to the 

 bilateral cleavage ot tunicates, where the conditions of cleavage resemble 

 those of Lepas — has with some necessary modifications been followed. 

 The cells of the four-cell stage are designated a, b, c and d iu the usual 

 order, a being the left anterior cell. An exponent iudicatQS the number 

 of the generation, starting Avith the ovum as the first, e. g. a', 5^ etc. 

 A second exponent is used to distinguish a cell from other cells of the 

 same generation and derivation, e. g. a*'\ a*-', a*-^, etc. In assigning 

 the second exponent I have followed in part suggestions made by Kofoid 

 ('94) and put into practice by Castle ('96). In cases of equatorial 

 division the odd numbers have been applied to the cells nearer the ve(/e- 

 tative pole, and the even to those nearer the animal pole. Thus of the 

 cells in the four-cell stage a^ divides, forming a*-' which is nearer the 

 vegetative, and a*-^ which is nearer the animal pole, while its sister cell, 

 b^, forms 5*-^ and i**^ (see Plate 4, Figs. 34-38). In later stages, where 

 cells do not divide equatorially, but parallel to the sagittal plane, the odd 

 exponent has been applied to the cell lying nearer that plane. In cases 

 where a cell lies in the sagittal plane and undergoes division in the same 

 plane, the daughter cell on the i-ight side of that plane is designated by 

 the odd exponent. Whenever cells divide transversely to the chief axis 

 of the embryo, the anterior cell is designated by the odd exponent. 



In determining the designation of cells, the rules given by Kofoid are 

 here applied to Lepas. The designation of any derivative of cells a, b, 

 c, d being given, the designation of mother cell or daughter cells can be 

 quickly determined. The first exponent indicating the generation of the 

 mother cell will, of course, be one less than that of the daughter cell. 

 The second exponent of the mother cell will be one-half of that of the 

 daughter cell, if that be an even number, and one-half the sum of the 

 second exponent plus one, if that be an odd number. Thus a*'^ and a*-' 

 are daughter cells of a'-\ Likewise, to determine the first exponent of 

 the daughter cells, add one to the first exponent of the mother cell ; to 

 determine the second exponent, multiply the second exponent of the 

 mother cell by two and the product is the designation to be applied to 

 the cell bearing the even number as exponent, while that product less one 

 designates the sister cell. Thus a*-*^ dividing forms a*-^^ and a^-". 



A summary of the important points in the cell-lineage of Lepas is given 

 in a table in connection with the general summary. 



