BIGELOW: EARLY DEVELOPMENT OF LEPAS. 125 



ectoblastic micromeres seen in the eight-cell stage of such eggs as have 

 four macromeres resulting from the quartet-forming (first and second) 

 cleavages. The micromeres of Lepas are, then, according to this view, 

 to be regarded as equivalent to quartets of micromeres, while the single 

 yolk-macromere equals a quartet of macromeres. It must be recognized 

 that there are great, perhaps irreconcilable, differences between the de- 

 velopment of the cirripedes and that of annelids and mollusks, and that 

 consequently, the above comparisons might be extreme, if they were to 

 be used as evidence of the existence of cell-homologies. At present it is 

 possible simply to compare the order of cleavages involved in segregating 

 the germ-layers. 



A similar relation in cleavage occurs within the group of the Cirripe- 

 dia. Van Beneden ('70) showed that in the Rhizocephalan genus Saccu- 

 lina, the first and second cleavages divide the egg into a quartet of 

 yolk-bearing macromeres, all containing entoblast, from which a quartet 

 of ectoblastic micromeres is separated by the third cleavage in the 

 formation of the eight-cell stage. This is exactly the order of cleavages 

 in the eggs of annelids and mollusks. In Sacculina, then, the first 

 segregation of ectoblast occurs two cleavages later than in Lepas, in 

 which there is precocious segregation of ectoblast. In Sacculina the 

 first and second cleavages divide the egg into four yolk-bearing macro- 

 meres, each containing entoblast and ectoblast, and the segregation of 

 the primary germ-layers begins at the third cleavage ; but in Lepas 

 the segregation begins at the first cleavage without subdivision of the 

 egg into four quadrants. Comparing the four-cell stage of the two 

 genera, the entoblast in Lepas is all concentrated into one of the 

 four cells each of which in Sacculina contains entoblast. According to 

 this view the first cleavage of Lepas corresponds to the third of Saccu- 

 lina so far as the first segregation of ectoblast is concerned. Whether 

 the first micromere of Lepas is homologous with the quartet of micro- 

 meres in Sacculina cannot be determined until the fate of those cells is 

 traced in the latter genus. There is reason for inferring that in Saccu- 

 lina other quartets of ectomeres are cut off from the yolk-macromeres 

 and added to the ectoblast. This must be settled before any further 

 conclusions can be drawn. The final result of the development the 

 Nauplius is similar in Lepas and in Sacculina. A comparison of the 

 cell-lineage of the two genera may be expected to yield some results 

 bearing on the suggestion that possibly the micromeres (a 2 , c 8 , c? 4 * 2 ) of 

 Lepas may be equivalent to quartets of ectoblastic micromeres in Saccu- 

 liiia, and possibly to those in more distantly related forms. These are 



