S YSTEMA TIC EMBR YOLOG Y. 



107 



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. 



FIG. 60. THKEE 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 (m) has become con- 

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



in. mouth; al. alimentary canal; ae. archenteron; bl.p. blastopore; pv. perivisceral 

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



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 invagi- 

 nations. They give rise respectively to the oral and anal extremities 

 of the alimentary tract. 



BIBLIOGRAPHY. 



(107) K. E. von Baer. " Ueb. Entwicklungsgeschichte d. Thiere.'' Konigsberg, 

 18281837. 



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



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

 Vide also Translation. Elements of Comparative Anatomy. Macmillan and Co., 1878. 



(no) E. Haeckel. Studien z. Gastraa-Theorie. Jena, 1877, and also Jenaische 

 Zeitschri/t, Vols. vm. and ix. 



(in) E. Haeckel. Schopfungsgeschichte. Leipzig. Vide also Translation. The 

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



(112) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthropo- 

 geny (Translation). Kegan Paul and Co., London, 1878. 



(113) Th. H. Huxley. The Anatomy of Invertebrated Animals. Churchhill, 1877. 



(114) E.R. Lankester. "Notes on Embryology and Classification." Quart. J. of 

 Micr. Science, Vol. xvn. 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. 



