CHAPTER VII 



THE EXTERNAL FEATURES OF THE BODY 



The preceding chapters have dealt with the ontogenetic evolution 

 of the various organ systems of the vertebrate body. The present 

 chapter will sketch in outline the development of the external 

 characteristics in so far as these have not already been referred to.' 



(1) Development of General Foem. — The groups of vertebrates 

 in which the egg is typically holoblastic will be considered first. 



Crossopterygii. — Of the two surviving genera Polypterus alone 

 has been studied (Graham Kerr, 1907) and the main features in the 

 development of its body-form may be gathered from ah inspection of 

 Fig. 197. 



It will be seen that the head-end of the embryo is the first to 

 project freely above the general surface of the body (Fig. 197, A). 

 The tail projection soon however makes its appearance (Fig. 197, B) 

 and during subsequent stages grows much more actively in length, 

 the embryo soon assuming a somewhat tadpole-like shape, with a 

 laterally compressed hinder region and a rounded swollen anterior 

 region formed by the main part of the yolk-laden egg. Two organs, 

 the cement-organ (c.o) and the external gill (e.g), make their appear- 

 ance as slight bulgings of the surface at a very early stage. During 

 subsequent stages the hinder, laterally compressed region grows 

 rapidly at the expense of the mass of yolk which becomes con- 

 sequently reduced in volume and at the same time loses its spherical 

 shape so that it projects less prominently. It will be noticed that 

 during the later stages the post -anal region grows particularly 

 actively, the anus thus coming to lie at a relatively greater and 

 greater distance from the hinder end of the body a;id giving rise to 

 a rapidly growing true "tail" region. During the later stages (Fig. 

 197, D, E, F) the head undergoes much increase in length, its active 

 forward growth beginning about stage 31. The mouth is at first 

 widely gaping (Fig. 197, E) but by stage 34 the mouth-hinge becomes 

 functional and it can be closed. By stage 36 the anterior swelling 

 due to the yolk has practically disappeared. 



Actinopterygii. — Amongst the Actinopterygian fishes the 

 Ganoids which still retain the holoblastic segmentation show very 



429 



