210 GENERAL BIOLOGY 



Study and diagram segmentation and gastrulation stages 

 and the main features of formation and closure of the neural 

 groove and the development of gill slits and gills as seen in 

 external views of the specimens. The labor of drawing will 

 be lightened if uniform circles be drawn mechanically for 

 the earlier stages, and cut out forms be used for outlines 

 of the later ones; or, if printed or otherwise duplicated 

 outline figures be furnished. This may be supplemented 

 by microscopic projection of egg sections. 



The record of the study will consist in the series of dia- 

 grams made. 



The salamander, a typical vertebrate. Because of its 

 primitive structure, the salamander serves well for intro- 

 duction to the study of the vertebrates. The parts we 

 have found in it we would find in all the others, only modi- 

 fied in form. As it develops, so do the others, in the main; 

 in all, the principal organs are laid down in like manner 

 and have like relations to the germ layers, and to each 

 other. Neural tube, notocord and gill arches are formed 

 in all. A two chambered heart and a fish-like cir- 

 culation develop first, and a pronephros, before the true 

 kidney; but some develop much farther than the sala- 

 mander, and along very peculiar lines. The salamander 

 ends its improvement of circulatory apparatus with two 

 aortic arches doing precisely the same work; but in the 

 higher groups of vertebrates, birds and mammals, we 

 find one of these arches has been done away with and the 

 other one does the work alone; the right one has been 

 retained in birds, the left one in mammals (fig. 1300). 

 A further improvement in birds and mammals is found 

 in the four chambered heart, which with better de- 

 veloped lungs makes possible a complete double circu- 

 lation of the blood (fig. 1 3 5), all the blood passing through 



