124 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
So far it has not been shown conclusively that the mesoblast of anne- 
lids has a like double origin, but the studies of Wilson (98) make it 
appear probable that in the annelid egg there is mesoblast of ectoblastic 
origin, which is comparable to the “secondary mesoblast ” or “larval 
mesenchyme” of mollusks.? 
It must be understood that, in offering the following suggestions of 
some resemblances between the cleavage of Lepas and the forms above 
mentioned, it is not here claimed that any cell homologies exist. Our 
knowledge of this subject is not as yet sufficiently extensive to warrant 
any decision for or against such a conclusion. 
The fact that in Lepas the ectoblast is separated from the mes-ento- 
blast by three successive cleavages, while the fourth separates the pri- 
mary mesoblast from the entoblast is, at least, an interesting coincidence. 
The double origin of mesoblast is another point of resemblance, for in 
Lepas, as in gasteropods, lamellibranchs and probably annelids also, the 
ectoblast is a second source of mesoblastic cells. 
In one important respect there seems to be a wide difference between 
the cleavage of Lepas and that of annelids and mollusks; for in these 
latter groups there are three quartets of ectoblastic micromeres formed 
by as many successive cleavages of four macromeres, whereas in Lepas 
there are not three quartets of cells but three cells formed in the same 
order of cleavage. In the annelids and mollusks the first segregation of 
ectoblast from entoblast is represented by the upper four cells (first 
quartet of micromeres) of the eight-cell stage, formed by the third cleav- 
age, whereas in Lepas the first segregated ectoblast is one of the two 
cells formed by the first cleavage. Stated in other terms, in annelids 
and mollusks, unlike Lepas, the first and second cleavages are not 
directly concerned with the segregation of ectoblast from entoblast, but 
they divide the egg into a quartet of macromeres, each containing ento- 
blast, from which in succession three quartets of ectoblastic micromeres 
are separated. In Lepas the segregation of ectoblast begins, as it were 
precociously, without the previous division of the entoblast into a quar- 
tet of cells. As a result of this there is in Lepas one entoblastic macro- 
mere instead of four, as in annelids and mollusks, and single micromeres 
appear to represent quartets. So far as the order of cleavage involved 
in the segregation of the primary germ-layers is concerned, the first 
micromere (ab?) of Lepas apparently corresponds to the first quartet of 
1 Since this paragraph was written, several investigators have given support to 
the suggestion that there is a double origin of the mesoblast in annelids, See 
Treadwell (: 01, p. 427), Wilson (:01, p. 891) and Torrey (: 02, p. 576). 
