vii EXTERNAL FEATURES OF DIPNOI 435 



head. The posterior part of the body now becomes laterally com- 

 pressed, it grows rapidly in length and the larva assumes a somewhat 

 tadpole-like form — the apparent " tail " being at first bent ventrally 

 (Fig. 200, C). The anus is situated close to the tip of this portion 

 of the body, therefore it is, strictly speaking, not tail but rather 

 posterior trunk-region. About this period hatching takes place. 

 The tail-like hinder region now straightens out (Fig. 200, D) and 

 grows rapidly in length, the growth being at first mainly pre-anal 

 and the true tail-region developing later. As in Crossopterygians 

 the tail is throughout protocercal. As in Polypterus again the head- 

 region for a considerable period shows no active growth in length : it 

 is not until about stage 31 (Fig. 200, E) that its growth becomes active 

 and the head-region begins to develop the modelling of its definitive 

 features. The external gills grow actively in length after hatching : 

 each develops a double row of pinnae along its external margin and 

 eventually all four become fused together at their bases. They reach 

 their maximum about stage 35 and thereafter undergo a process of 

 atrophy resulting in their complete disappearance. The limbs make 

 their appearance about stage 31, each as a little knob bearing a 

 striking resemblance to the first stage of an external gill. The 

 cement organ increases in size forming a large cushion-like and very 

 conspicuous organ in the larva of stages 32-34 (Fig. 200, F). Eventu- 

 ally it shrivels up and disappears without leaving a trace behind. 



In Protopterus as already mentioned the general features of 

 development agree very closely with those of Lepidosiren. 



Of Ceratodus developmental material was obtained by Caldwell 

 in 1884 and by Semon in 1891. Semon's material has formed 

 the basis of a long series of investigations by himself and others 

 which together constitute an important contribution to Vertebrate 

 morphology (Semon, 1893-1913 ; 1901*). 



While the general modelling of the body shows a general 

 resemblance to that of Lepidosiren and Protopterus there are certain 

 well-marked differences in detail. Perhaps the most striking of 

 these is that the head-region shoots ahead in its development and 

 grows actively in length so as to project freely in front of the yolk 

 at a much earlier period than in the other genera (Fig. 201, A, B). 

 Again the main mass of yolk undergoes a more uniform process of 

 lengthening so that it assumes a somewhat spindle-like form and 

 allows the body as a whole to become slender and "fish "-like, the 

 " tadpole " shape due to the persisting spherical mass of yolk in 

 Polypterus or Lepidosiren being here absent. During the later 

 larval stages the divergence of Ceratodus from the other two Lung- 

 fishes towards the more typical fish condition becomes marked by 

 the paddle-like form of the limbs and the much greater development 

 of the median fin round the hinder end of the body. It will be 

 noticed also that two conspicuous features of the young Lepidosiren 

 or Protopterus — the Cement organ and the external gills — are 

 completely absent in Ceratodus. 



