258 HISTORY OF THE HUMAN BODY 



sorbing, and assimilating the nutritive qualities of the food, 

 and thus formed the digestive cavity, the first portion of the 

 organism to differentiate as a distinct system. This digestive 

 cavity, or gastroccele, remains in the lower invertebrates as a 

 blind cavity with but a single opening, and first among the 

 worms (Vermes), it becomes converted into a complete canal 

 by the formation of an anal orifice, thus obviating the necessity 

 of employing the same orifice for both the intaking and the 

 expulsion of the contents of the cavity. 



A further advance in the development of the endodermic 

 portion of the organism is seen in the higher invertebrates 

 (articulates, echinoderms, etc.), and in the vertebrates, where 

 certain lateral diverticula become divided off from the primary 

 alimentary canal, and form a definite body cavity, the coeloin, 

 so that the ultimate alimentary canal of these animals is but 

 a part of the canal of the lower organisms. In vertebrates 

 the canal suffers a still farther loss by the formation and later 

 separation of the notochord. Another departure from the 

 primary condition is seen in the mouth and anus of vertebrates, 

 which are shown by their development not to be homologous 

 with the similarly named cavities of lower forms but new 

 formations, involving other morphological relations, and 

 formed by contributions from the ectoderm. In the develop- 

 ment of many invertebrates the primary mouth of the gastrula 

 becomes the permanent one of the adult organism and an 

 anus is formed by continuing the blind end of the gastrular 

 invagination until it meets the surface ectoderm at a point 

 opposite that of the mouth ; in the vertebrate embryo, how- 

 ever, the gastrular mouth lies postero-dorsally with reference 

 to the future animal and thus bears no relation to eithei 

 mouth or anus of the perfected form (Fig. 71, A). Durinj 

 the development of the nervous system, however, there comes 

 a curious and inexplicable connection between the lumen 

 of the neural tube and the gastrular mouth, which effects a 

 temporary connection between this cavity and that of the 

 gastrocoele through the so-called neur enteric canal (Fig. 71, 

 B), but this connection is only transitory and the entire struc- 



