136 bulletin: museum of compakative zoology. 



ADDENDUM. 

 By E. L. Mark and W. E. Castle. 



To avoid any misunderstanding we wish to state that the opinions 

 expressed by Dr. Bigelow regarding "quartet" cleavage are not wholly 

 shared by us. Lepas seems to us a good example of modified " quartet " 

 cleavage, and for that reason we think the quartet nomenclature has 

 more than mere convenience in its favor. To be sure, the quadrants in 

 Lepas are not symmetrical, but perfect symmetry is rarely met with in 

 quartet cleavage. So far as we recall, complete symmetry of the quad- 

 rants is found only in platodes. The condition there realized may be 

 considered primitive, all four quadrants sharing equally in the produc- 

 tion of ectoblast, mesoblast, and endoblast (see Wilson, '98). One 

 modification of this primitive symmetry is found in annelids and 

 moUusks, another in rotifers and cirripedes. 



In the first-named groups the mesoblast is segregated, more or less 

 completely, in quadrant d, while the endoblast remains distributed 

 among all four quadrants. In the rotifers (see Jennings, '96) the en- 

 doblast is segregated in quadrant d, precisely as in Lepas, yet the cleav- 

 age progresses in perfect quadrant symmetry through at least the first 

 eight cell-generations, even though, to realize this symmetry, so-called 

 " mechanical laws of cleavage " are repeatedl}' transgressed. The origin 

 of the mesoblast in rotifers remains uncertain, but in Lepas, as Dr. 

 Bigelow clearly shows, the mesoblast arises from all four quadrants. 

 An examination of his table of cell-lineage (p. 135) shows other un- 

 mistakable evidences of quadrant symmetry in Lepas. 



L The first-formed definitive ectomeres — which are also the first 

 cells to be differentiated for a particular germ-layer — arise sym- 

 metrically and synchronously from all four quadrants. They are the 

 four dorsal cells of the eiglit-cell stage, namely, a*'^, 6* ^ c*"^, and d'^-'^. 

 They correspond with what in polyclads, annelids, and mollusks have 

 been called the " first quartet of micromeres," which in these forms, as 

 in Lepas, are always the first ectomeres to be differentiated. 



2. At the sixteen-cell stage, in Lepas, the mesoblast is included in 

 corresponding blastomercs (a■^•^ b''-'^, c^'^, d^'^) in all four quadrants. 



