. Journal of morphology. it has been shown that cells of the same lineage,even in widely separated species, undergo, as a rule, the sameultimate differentiation. The mesoblast was the first of the germ-layers to be tracedback to a single cell. Kowalevsky (No. 52) in 1871 showed thatthe mesoblast of Lumbricus could be traced back to twoposterior pole cells, derived from the entoderm, which buddedoff anteriorly, a large number of small cells thus forming amesoblastic germ band. He postulated a double source forthe mesoderm of Euaxes: the larger part came from two largecells derived from the post
. Journal of morphology. it has been shown that cells of the same lineage,even in widely separated species, undergo, as a rule, the sameultimate differentiation. The mesoblast was the first of the germ-layers to be tracedback to a single cell. Kowalevsky (No. 52) in 1871 showed thatthe mesoblast of Lumbricus could be traced back to twoposterior pole cells, derived from the entoderm, which buddedoff anteriorly, a large number of small cells thus forming amesoblastic germ band. He postulated a double source forthe mesoderm of Euaxes: the larger part came from two largecells derived from the posterior macromere ; the rest wasderived from two small cells, derivatives of the two lateralmacromeres. Rabl (No. 25) formulated his results in 1876 asfollows: Das mittlere Keimblatt entsteht also nach unserenAuseinandersetzungen aus zwei, am Mundrande der Gastrulagelegenen Zellen, deren Verwandschaft zu den Zellen desinneren Blattes eine viel innigere ist, als zu jenen des aus- No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 33. 34 LILLIE. [Vol. X. seren. The origin of the mesoblast, from two bilaterallysymmetrical cells, has often since been described. ProfessorWhitman (No. 6i, 1878) was the first to show that there was aperfectly definite cell history and origin from single cells forother structures of the adult. He demonstrated that, inClepsine, not only the mesoblast, but the ventral nerve cordand the trunk nephridia could be traced back to single cells ;and further, that the cells representing these structures werethe product of the posterior macromere of the four-cell stage,which was thus the representative in this stage of the wholetrunk. In 1879 Rabls paper on Planorbis appeared. Heshowed, for the first time, that the whole ectoderm was formedin a series of three cleavages each, from four basal (No. 35), 1882, described three generations ofectomeres in Neritina, and derived the mesoblast fromthe fourth cleavage of the posterior macromere. Duringthe last twe
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