BIGELOW: EARLY DEVELOPMENT OF LEPAS. 75 



ence to the conditions in the developing eggs of annelids and mollusks, 

 are dominated by the conception of cells cleaving in sets of fours or quar- 

 tets. The system of Blochmann ('81) and its successors have, with few 

 exceptions, been applied to eggs in which a quartet of macromeres (in a 

 morphological sense) is formed by the first two cleavages, and by later 

 cleavages these give rise to successive quartets of micromeres. In all 

 the annelids and mollusks in which the cell-lineage has been determined 

 with certainty, the cells of the four quadrants (a, b, c, d) formed by the 

 first two cleavages are equivalent, in that each cell contains a portion of 

 the two primary germ-layers, ectoblast and entoblast. The mesoblast 

 is not so distributed with reference to the quadrants. It will be shown 

 in this paper that the four-cell stage of Lepas is not a quartet of equiva- 

 lent cells so far as the two primary germ-layers are concerned. Whereas 

 in the annelidan and molluskan eggs each cell of the four-cell stage con- 

 tains both ectoblast and entoblast, in Lepas three of these cells (a, b, c) 

 contain ectoblast but no entoblast ; and the fourth cell (d) contains both 

 ectoblast and all the entoblast. In the annelids and mollusks the cells 

 of the first quartet of micromeres (eight-cell stage) contain the ectoblast 

 which is first separated from the entoblastic macromeres ; but in Lepas 

 one of the cells of the two-cell stage is the first ectoblast to be separated 

 from entoblast. 



Enough has been said, in anticipation of the account of the cleavage, 

 to make it evident that the well-known quartet systems of nomenclature 

 would not have their usual significance as indexes of homologies, if 

 applied to the cleavage of Lepas, for the cells of the four-cell stage in 

 annelids and mollusks are apparently not comparable with the cells of 

 the same stage of Lepas, which would be given the same designations. 

 However, a quartet system has been employed for the purposes of this 

 paper, for the reason that it is convenient and familiar. The above 

 statements will show that the system has not been used here with a view 

 to indicating by it homologies with which it has become associated in its 

 application to the spiral cleavage of annelids and mollusks. As far as 

 regards the cirripede egg, the known facts do not seem to me to warrant 

 the interpretation that cleavage occurs in cells grouped as quartets in the 

 sense in which the term is applied to spiral cleavage ; and while the 

 notation of a quartet system has been adapted to the purposes of this 

 paper, the term " quartet " has not been applied in description as desig- 

 nating groups of cells in the cleaving egg of Lepas. 1 



1 See Addendum by E. L. M. and W. E. C. (p. 136) following the General 

 Summary. 



