130 INTRODUCTION. 



FIG. 60. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after 

 Biitschli 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 com- 

 mencing 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 (/) has become con- 

 tinuous with alimentary tract, and the blastopore has become closed. 



m. mouth ; al. alimentary canal ; ae. archenteron ; bl. p. blastopore ; pv. perivis- 

 ceral cavity ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs. 



the stomodaeum and proctodaeum, derived from epiblastic 

 invaginations. They give rise respectively to the oral and anal 

 extremities of the alimentary tract. 



BIBLIOGRAPHY. 



(107) K. E. von Baer. " Ueb. Entwicklungsgeschichte d. Thiere." Konigs- 

 berg, 18281837. 



(108) C. Glaus. Grttndzuge d. Zoologie. Marburg und Leipzig, 1879. 



(109) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878. 

 Vide also Translation. Elements of Comparative Anatomy, Macmillan and Co., 

 1878. 



(110) E. Haeckel. Studien z. Gastraa-Theorie. Jena, 1877, and vt\sx> Jenaischc 

 Zeitschrift, Vols. vill. and IX. 



(111) E. Haeckel. Schopfungsgeschichte. Leipzig. Vide also Translation. 

 The History of Creation. King and Co., London, 1876. 



(112) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation. Anthro- 

 Pogeny (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 Entvricklungsgesch. d. 

 Mensfhen it. d. Thiere. Leipzig, 1833. 



