124 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



So far it lias 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. 1 



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 eutoblast 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 quarters 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 (ai 2 ) 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). 



