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 (aé?, c*, d*?) of 
Lepas may be equivalent to quartets of ectoblastic micromeres in Saccu- 
lina, and possibly to those in more distantly related forms. These are 
