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 Nussbaura 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 later 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 cleav- 

 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 l)lastoderm and yolk-endoderm much variation has been 

 shown to occur, so much indeed that the process may be termed irregu- 



