134 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



separated from the yolk-bcaring macromcrc, and the fourth cleavage 

 separates the primary mesoblast from the yolk-eutoblast. Thus, in the 

 sixteen-cell stage the entobhxst is completely separated from the other 

 germ-layers. 



Mesoblast originates both from entoblast (fourth cleavage) and from 

 ectoblast (sixth cleavage). The mesoblast derived from ectoblast 

 C secondary mesoblast ") forms a large part at least of the mesen- 

 chyme of the Nauplius. The fate of the primary mesoblast (entoblastic 

 mesoblast) has not been distinguished from that of the " secondary 

 mesoblast" (ectoblastic mesoblast). 



The blastoderm grows over the yolk-bearing entoblast, usually closing 

 the blastopore after the sixth cleavage. In cases where the yolk-mass is 

 very large, the closing of the blastopore may not occur until the suc- 

 ceeding cleavage. But in all cases the blastoderm is formed from de- 

 rivatives of three and only three micromeres {aP, c', d*'^), which are 

 cut off in the first three cleavages. 



The yolk-macromere of the sixteen-cell stage has been traced to the 

 mesonteron. All the evidence supports entirely the interpretation that 

 after the fourth cleavage the yolk-macromere is purely entoblastic. 



The irregularity and variability which authors have ascribed to the 

 cleavage of cirripedes do not normally exist in the case of Lepas. The 

 origin, relative position, and fate of all cells of all cleavage stages have 

 been shown to be constant, definite, and " determinate " so far as the 

 formation of germ-layers is concerned. In later stages specific areas of 

 cells, known to be of definite origin, enter into the formation of particu- 

 lar organs. It is therefore probable that the cells in cleavage stages 

 bear a definite and constant relation to future organs. 



The chief points in the cell-lineage and their relation to the formation 

 of the germ-layers are summarized in the accompanying table. 



Describing the formation of the germ-layers of Lepas in general terms, 

 there is no conflict with most existing accounts of the development of 

 other Crustacea ; in the absence of complete records of the cell-lineage 

 in other Crustacea, it is not possible to compare the details with cer- 

 tainty (see Summary, p. 133). 



