Papers . ondition in Antedon, in which Seeliger foundno free cells in the blastocoele until after the invagination of the endo-derm. Mortensen (1920), in his study of the development of oneof the viviparous Crinoids, Isornetra vivipara, found a cleavage ofthe kind typical of Arthropods; there is a division and migration ofnuclei before any cell-walls appear; ectoderm and endoderm aredifferentiated in place, no invagination occurring. There are nocells in the space between ectoderm and endoderm, but some yolkgrains may be seen lying in this cavity. Guthrie and Hibbard (1919) have given a summar
Papers . ondition in Antedon, in which Seeliger foundno free cells in the blastocoele until after the invagination of the endo-derm. Mortensen (1920), in his study of the development of oneof the viviparous Crinoids, Isornetra vivipara, found a cleavage ofthe kind typical of Arthropods; there is a division and migration ofnuclei before any cell-walls appear; ectoderm and endoderm aredifferentiated in place, no invagination occurring. There are nocells in the space between ectoderm and endoderm, but some yolkgrains may be seen lying in this cavity. Guthrie and Hibbard (1919) have given a summary of the factsknown regarding the time of formation of mesenchyme in the various 12 Hybridization of Echinoids. classes of Echinoderms. To the list given in the paper cited shouldbe added Mortensens contributions on Crinoids and that of Oshima(1918, 1921) on the Holothurian Cucumaria echinata, in which themesenchyme cells are described as migrating into the blastocoelebefore the beginning of Fig. 5a, Skeletal rods from fixed and mounted Cidaris larva. 6 days. X , Cidaris larva, 2 weeks old, fixed and mounted. Camera sketch. X , Camera sketch of Uving Cidaris 9 X LytechinuscT gastrula. 24 hours. X , Camera sketch of living Cidaris 9 X Lytechinus cf gastrula. 24 hours. X , Camera sketch of living Cidaris 9 X Lytechinus d^ gastrula. 40 hours. X 220. Mortensens (1920) studies have shown that there is diversity inthe manner of formation of mesenchyme in the Crinoids, a factwhich our knowledge of the development of three species of Antedonhad failed to reveal. These observations do not affect the conclusionsthat I have drawn regarding the primitive nature of the developmentof Cidaris. Cidaris irihuloides, in the estimation of those qualified to speak onthe systematic position of echinoids, is primitive. This fact suggests Hyhridization of Echinoids. 13 the probability, although it does not absolutely establish the proof,of the conclusion that th
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