'Studien zur Gastræa-Theorie.' Jena, 1874. The transformation of the single primitive egg-cell by cleavage into a globular mass of cells (Morula)--which latter, becoming hollow (and then known as the Blastula), turns ultimately by invagination or by delamination into the Gastrula--is a series of processes which applies to all Metazoa. The Gastrula is, therefore, the ancestral form of the Metazoa; and the Gastræa-theory, founded by Haeckel, throws light, on the one hand, upon the mystery of the phyletic connection of the various animal groups, while, on the other hand, it connects the Metazoa, or multicellular organisms, with the lowest Protozoa. We come to this conclusion becaues the Gastrula arises from and passes through stages which exist as independent, permanent organisms among the Protozoa. Needless to say this Gastræa-theory has been violently attacked in detail, with the result that various modifications of the Gastrula, until then undreamed of, have become known. 'Monographie der Medusen.' Jena, 1879-81. With 72 coloured plates. 'Reports on the Scientific Results of the Voyage of H.M.S. _Challenger_.' With 230 plates: 1. Deep-sea Medusæ. 1881. 2. Radiolaria. 1887. 3. Siphonophoræ. 1888. 4. Deep-sea Keratosa. 1889. A short holiday spent on the coasts of the Red Sea produced the volume 'Arabische Korallen' (Berlin, 1876); and a longer trip to Ceylon has been described in 'Indische Reisebriefe,' of which the third edition appeared in 1893. The English translation (1883) is entitled 'A Visit to Ceylon.' 'Monism as connecting Religion and Science: the Confession of Faith of a Man of Science.' 1894. Haeckels latest work is the 'Systematische Phylogenie' (Berlin, 1896), three volumes dealing with Protistæ and Plants, Invertebrata and Vertebrata. They contain the author's views on the natural system of the organic world, both living and extinct. Notable in the work are the many reconstructions of ancestral forms which, provided Evolution is true, must have existed--hypothetical until they, or something like them, are found in a fossil state. Everybody who works systematically, and upon the basis of Evolution, does, sometimes unconsciously, reconstruct such links, although he may perhaps not see the necessity, or have the courage to fix his vision, by assigning to it all those attributes or characters which are indicated by deductions from comparative anatomy, palæontology, and embryology. THEORY OF CELLS. The vegetable cell was discovered by _Schleiden_, Professor of Botany at Jena, in 1838. Next year _Schwann_ found the animal cell. In 1844 _Koelliker_ discovered that the egg cell, by division and multiplication, becomes an aggregation--a heap of new cells. In 1849 _Huxley_ found the two primary layers (observed long before by _Pander_ and _Baer_ in the chick) also in certain Invertebrata, the Medusæ; and he called these layers 'ectoderm' and 'endoderm' respectively.