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, 
ae | — = TT = | 
q ks ® mr kg & sao wae ee ® £ a @ A&A wv 
Fic. 45.—Ideal primitive vertebrate, seen from the left side. (After 
Hackel.) a, nose; au, eye; g, ear; md, mouth; &s, gill-openings; 
x, notochord; mr, spinal tube; 4g, gill-vessels; 4, gill-intestine; hz, 
heart ; es, muscles: ma, stomach ; v, intestinal vein; c, body-cavity ; 
a, aorta; Z, liver; @, small intestine; e, ovary; 4%, testes; 2, kidney 
canal; af, anus; /f, true or leather-skin; 0.2, outer-skin (epidermis) ; 
J, 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 
