Embryology. 



143 



followed by the Vertebrata — observing merely that, if 

 space permitted, the same principles of progressive 

 though diverging histories of evolution would equally 

 well admit of being traced in all the other sub-king- 

 doms which have just been named. 



In order to trace these principles in the case of the 

 Vertebrata. it is desirable first of all to obtain an idea 

 of the anatomical features which most essentially dis- 

 tinguish the sub-kingdom as a whole. The following, 



\s *■ mr ](„ 



Fig. 45. — Ideal primitive vertebrate, seen from the left side. (After 

 Hackel.) na, nose; au, eye; g, ear; md, mouth ; ks, gill-openings; 

 X, nctochord ; mr, spinal tube ; kg, gill-vessels ; k, gill-intestine ; kz, 

 heart ; ms, muscles ; ma, stomach ; v, intestinal vein ; c, body-cavity ; 

 a, aorta; /, liver; </, small intestine; e, ovary; h, testes; «, kidney 

 canal ; af, anus ; Ih, true or leather-skin ; oh, outer-skin (epidermis) ; 

 f, skin-fold, acting as a fin. 



then, is what may be termed the ideal plan of verte- 

 brate organization, as given by Prof Hackel. First, 

 occupying the major axis of body we perceive the 

 primitive vertebral column. The parts lying above 

 this axis are those which have been developed from 

 the ectoderm and mesoderm — viz. voluntary muscles, 

 central nervous system, and organs of special sense. 

 The parts lying below this axis are for the most part 

 those which have been developed from the endoderm 



