136 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
ADDENDUM. 
By E. L. Marx anp 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 
mollusks, 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 repeatedly 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. 
1. 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 eight-cell stage, namely, a*?, b*?, 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 blastomeres (a®**, 6°-?, c®:?, d°-*) in all four quadrants. 
