EARLY DEVELOPMENT OF AMPHIOXUS. 47 



vessels and lymphatics, the tubes through which substances are carried 

 from one part of the body to another, the blood and lymph also which are 

 the vehicles for these substances, all are mesodermal in origin. The organs 

 of excretion too arise from this intermediate layer. The reproductive 

 organs, growth centers of the germ cells., originate here. It is not difficult 

 to see, therefore, that in the higher and more complex animal forms many of 

 the activities of the ectodermal and entodermal derivatives which are cor- 

 related with response to external stimuli and with alimentation are made 

 possible by structures elaborated from the mesoderm. 



While Amphioxus is not a true vertebrate because it never acquires a 

 vertebral column, yet we may observe in it a relatively simple arrangement 

 of structure which foreshadows the fundamental vertebrate organization. 

 After the development of the mesoderm and ccelom the embryo as a whole 

 obviously comprises a tube within a tube; the gut, extending from mouth 

 to anus, is the inner tube, the body wall is the outer tube, and the two are 

 separated by the ccelom or body cavity. This is a typical vertebrate char- 

 acteristic. The neural tube or central nervous system, situated in the dorsal 

 body wall, is another feature which links Amphioxus with the vertebrates. 

 The notocord which is regarded as the axial supporting structure in Am- 

 phioxus appears also in higher animal forms. In the true vertebrates the 

 notocord is not transformed into the axial skeleton which is the chief longi- 

 tudinal supporting skeleton, but the axial mechanism is built around the 

 notocord. Another impressive attribute of the vertebrates is the series 

 of mesodermal somites, although it must be remembered that this is not 

 exclusively chordate property, for some of the invertebrates, for instance the 

 worms, possess it. This transverse segmentation, or metamerism, affects 

 not only the mesoderm and certain of its derivatives but involves also struc- 

 tures that arise from ectoderm. In the vertebrates the units of the spinal 

 column, arising from the somites, maintain their integrity throughout the life 

 of the animal. The ribs and intercostal muscles are expressions of metamer- 

 ism. Many of the blood vessels are arranged segmentally. Even the 

 primitive kidney arises as a segmental organ. Among the ectodermal 

 derivatives, the nervous system reflects the metameric quality in the develop- 

 ment of the spinal nerves. Obviously many features of vertebrate organiza- 

 tion depend upon the principle of metamerism. 



References for Further Study. 



CERFONTAINE, P.: Recherches sur le development de 1' Amphioxus. Archives de 

 Biologie, tome 22, 1907. 



HATSCHEK, B.: Studien uber die Entwickelung des Amphioxus. Arbeiten aus dem 

 2007. Institut zu Wien, Bd. 4, 1881. 



