The development of isolated blastomeres in Bithynia tentaculata (Prosobranchia, Gastropoda).

The development of isolated blastomeres, separated at the first and second cleavage has been studied. After separation at the first cleavage, one half-embryo (CD) becomes swollen and balloon-like and develops adult organs such as eyes, tentacles, shell, operculum, etc. The other half (AB) is much more compact and never develops these adult structures. After separation on the alternate axis at the second cleavage to produce AD and BC halves, both halves could form all adult structures. These results indicate an equivalent organizing capacity of the C and D quadrants contrary to most spiralians, such as Ilyanassa, where the D quadrant alone exerts the major organizational influence. The experimental evidence is discussed in relation to the distribution of the cytoplasmic vegetal body, rich in RNA, which is passed to the CD blastomere in the polar lobe and then evidently equally distributed to C and D. Crampton (1896) first studied the development of isolated blastomeres in gastropods, using Ilyanassa, Urosalpinx and Anachis . He found that the separated cells underwent cleavage in a pattern typical of the part of the normal embryo from which they originated and not at all comparable to that of the whole embryo. A more extensive study on the developmental potencies of isolated blastomeres was carried out by Wilson ('04a,b) with eggs of the gastropod Patella and the scaphopod Dental ium. Wilson succeeded in rearing the isolated cells to a stage where larval structures appeared. He concluded that isolated cells can give rise to the same structures as they would have done as a part of the whole embryo. In addition, he established the important role of the polar lobe in the development of Dentaliurn, for after removal of this lobe at first cleavage a defective larva was formed without apical tuft and posttrochal region. Similar results were obtained by Rattenbury and Berg ('54) studying the development of lobeless embryos and isolated blastomeres of the bivalve Mytilus. Studies of isolated blastomeres in other spiralians have recently been reviewed by Cather ('71). The most penetrating studies on the significance of the polar lobe for development and on the potencies of indiJ. EXP. ZOOL., 186: 47-62 vidual blastomeres are those of Clement ('52, '56, '62) carried out with eggs of the mudsnail Ilyanassa. Clement showed that after removal of the polar lobe at the trefoil stage all distinctive features of the cleavage in the D quadrant are lost, so that cleavage becomes identical in all quadrants. In the lobeless larva no adult structures such as foot, eyes, tentacles, etc., originate. Clement's study of isolated blastomeres showed that adult structures only develop from the D macromere or combinations including it. The A, B, or C blastomeres always develop like lobeless embryos. Extending this study Clement removed the D macromere at successive stages. It appeared that removal of D, 1D and 2D has approximately the same consequences for later development as removal of the polar lobe, but after removal of 3D a larva with eyes, velum, shell and foot may develop. When only 4D is deleted even the heart and intestine are added to form a small but perfect larva. Hess ('56) succeeded in separating the blastomeres of the fresh-water prosobranch Bithynia tentaculata. He studied the cleavI Present address: Zoological Laboratory. Janskerkhof 3, Utrecht, The Netherlands. 2 Supported in part by institutional research grant IN-4OM to the University of Michigan from the American Cancer Society.