116 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
not the details of the cell-lineage been traced in Lepas, I should be led 
to describe in similar general terms the origin of the mesoblast. I infer 
from Nussbaum’s description that in Pollicipes the blastopore does not 
become closed as early as in Lepas. It seems probable that in Polli- 
cipes the primary and secondary mesoblast cells may undergo some 
divisions before they are forced beneath the overgrowing blastoderm. 
Such a process would have the appearance of the production of meso- 
blast from the blastoderm cells at the edge of the blastopore. 
In stages preceding gastrulation Nussbaum saw two large cells at the 
posterior pole, but he lacked material for following out their history. It 
seems probable that he saw the two primary mesoblasts which I have 
seen in the thirty-two-cell stage of Lepas. 
12. DETERMINATE CLEAVAGE, 
The small size and large number of cells make it impossible to de- 
termine the lineage of the individual cells of the embryo beyond the 
sixty-two-cell stage, and they cannot therefore be traced directly to 
particular organs of the Nauplius. However, the great regularity and 
constancy of preceding stages renders it extremely probable that the 
cells are destined for definite organs. Cells of definite origin have been 
traced to definite positions in the Jater cleavage stages. Careful ob- 
servation has given no evidence of changes in position of cells taking 
place after the completed segregation of the germ-layers. Indeed the 
beginning of irregularity is scarcely to be expected in such late and well 
differentiated stages of development. The regions of the embryo from 
which particular organs arise have been definitely traced to groups of 
cells of known lineage. There seems to be no reasonable doubt that the 
cells of the late cleavage stages are destined to enter into the formation 
of particular organs. The cleavage of Lepas is, then, an example of 
what Conklin (’98) has termed “ determinate cleavage.” 
The conclusions in the preceding paragraph on “ determinate cleay- 
age’ are widely at variance with those of all previous writers on cirri- 
pede development. The early development of the ova of cirripedes has 
always been regarded as irregular and indeterminate. Great variations 
have been said to occur. 
Groom (’94, p. 199) summarizes his study of the cleavage of various 
Cirripedia as follows: — “In describing the details of division of the 
cells of the blastoderm and yolk-endoderm much variation has been 
shown to occur, so much indeed that the process may be termed irregu- 
