BIGELOW: EARLY DEVELOPMENT OF LEPAS. 137 



The only essential difference among the quadrants in the mode of sep- 

 aration of the mesoblast is this : In quadrant d, cell d^'- is purely meso- 

 blastic ; but the corresponding cells in each of the other quadrants 

 contain mesoblast associated as yet with ectoblast, and the two are 

 not separated until the second later generation, that is, in the sixty- 

 four-cell stage. The earlier separation of the mesoblast in quadrant d, 

 as compared with the other quadrants, may be due to the relatively 

 greater bulk of the mesoblast in quadrant d. The mesoblast is really 

 partially segregated in quadrant d, — since that quadrant contains a 

 greater portion of mesoblast than any of the three remaining quadrants, 

 — while the endoblast is completely segregated in that quadrant. The 

 segregation of the mesoblast in quadrant d finds a parallel repeatedly in 

 mollusks and annelids ; that of the endoblast in the same quadrant is 

 paralleled in rotifers. 



Notwithstanding these coenogenetic modifications, the primitive quad- 

 rant-symmetry finds frequent expression in the cleavage of Lepas, a fact 

 to which the quadrant nomenclature clearly directs attention. 



It is true that in Lepas radial symmetry is replaced by bilateral sym- 

 metry considerably earlier than is the case in most annelids and mol- 

 lusks, and much earlier than in rotifers, but the difference is one of 

 degree rather than of kind. Cleavage in Lepas, as truly as in the other 

 forms mentioned, is at first radial, and only gradually becomes bilateral. 



