The mesoblast usually originates subsequently to the two primary layers. It then springs from one or both of the other layers, but its modes of origin are so various that it would be useless to attempt to classify them here. In cases of invagination it often arises at the lips of the blastopore (fig. 57 and 59), and in other cases part of it springs as paired hollow outgrowths of the walls of the archenteron. Such outgrowths are shewn in fig. 60, B and C at _pv_. The cavity of the outgrowths forms the body cavity, and the walls of the outgrowths the somatic and splanchnic layers of mesoblast (fig. C. _sp._ and _so._). The archenteron is in part always converted into a section of the permanent alimentary tract and the section of the alimentary tract so derived is known as the mesenteron. There are however usually two additional parts of the alimentary tract, known as the stomodaeum and proctodaeum, derived from epiblastic invaginations. They give rise respectively to the oral and anal extremities of the alimentary tract. [FIG. 60. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions. A. Represents the gastrula stage. B. Represents a succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the mesoblast. C. Represents a later stage in which the mouth involution (_m_) has become continuous with alimentary tract, and the blastopore has become closed. _m._ mouth; _al._ alimentary canal; _ae._ archenteron; _bl._ _p._ blastopore; _pv._ perivisceral cavity; _sp._ splanchnic mesoblast; _so._ somatic mesoblast; _ge._ generative organs.] BIBLIOGRAPHY. (107) K. E. von Baer. "Ueb. Entwicklungsgeschichte d. Thiere." Königsberg, 1828-1837. (108) C. Claus. _Grundzüge d. Zoologie._ Marburg und Leipzig, 1879. (109) C. Gegenbaur. _Grundriss d. vergleichenden Anatomie._ Leipzig, 1878. _Vide_ also Translation. _Elements of Comparative Anatomy._ Macmillan and Co., 1878. (110) E. Haeckel. _Studien z. Gastræa-Theorie_. Jena, 1877, and also _Jenaische Zeitschrift_, Vols. VIII. and IX. (111) E. Haeckel. _Schöpfungsgeschichte._ Leipzig. _Vide_ also Translation. _The History of Creation._ King and Co., London, 1876. (112) E. Haeckel. _Anthropogenie._ Leipzig. _Vide_ also Translation. _Anthropogeny_ (Translation). Kegan Paul and Co., London, 1878. (113) Th. H. Huxley. _The Anatomy of Invertebrated Animals._ Churchill, 1877. (114) E. R. Lankester. "Notes on Embryology and Classification." _Quart. J. of. Micr. Science_, Vol. XVII. 1877. (115) A. S. P. Packard. _Life Histories of Animals, including Man, or Outlines of Comparative Embryology._ Holt and Co., New York, 1876. (116) H. Rathke. _Abhandlungen z. Bildung und Entwicklungsgesch. d. Menschen u. d. Thiere._ Leipzig, 1833. CHAPTER IV.