172 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



dorsal mesentery (Fig. 97, D) but eventually the portion of this 

 mesentery containing the lung and dorsal to it becomes greatly 

 thickened from side to side and finally merges completely in the roof 

 of the splanchnocoele, so that in the adult condition the lungs lie 

 completely outside the body-cavity — between it and the vertebral 

 column. 



In Ceratodus (Gregg Wilson, 1901 ; Neumayr, 1904) the lung is 

 at first, as in the other two lung-fishes, ventral in position (Fig. 94, B) 

 but in this case the originally left lung, which in Lepidosiren and 

 Protopterus is for a time during development reduced in size, seems 

 to have disappeared almost entirely, being represented only by a small 

 and transient rudiment. Further detailed studies of the early stages 

 in the development of the lung of Ceratodus are much needed to 

 make clear the origin and fate of this vestigial left lung. But it 

 seems clear from what is already known that the monopneumatic 

 condition of Ceratodus has come about in evolution through the 

 suppression of the originally left lung. 



As the lung completes its development, its cavity becomes en- 

 croached upon by two median longitudinal ridge-like ingrowths, one 

 dorsal and the other ventral. It used to be supposed that these 

 marked an incipient division of the lung into a right and a left half 

 so as to bring about the condition seen in Lepidosiren or Protopterus 

 — the monopneumatic condition being supposed to be the more 

 nearly primitive. It will have been gathered from what has been 

 said that this point of view is no longer tenable and that the mono- 

 pneumatic condition of Ceratodus is to be looked on as secondary and 

 not primary. 



Crossopterygians. — Of the two surviving examples of the 

 Crossopterygian ganoids — the most archaic existing members of the 

 Ganoid-Teleostean stem — a few stages in the development of the lung 

 have been investigated in Polypterus (Graham Kerr, 1907). In the 

 earliest stage observed the lung-rudiment -was in the form of a mid- 

 ventral groove formed by an outgrowth of the pharyngeal lining 

 (Fig. 94, A, I). This groove becomes deeper and towards its posterior 

 end widens out ventrally so as to have a 1 -shape in transverse section. 



Posteriorly the lung-rudiment grows back into a pair of horn-like 

 projections — the rudiments of the right and left lung. These extend 

 backwards in the connective tissue of the splanchnopleure and they 

 very soon show a marked inequality in their rate of growth the left 

 lagging behind the right. As growth goes on this inequality becomes 

 more and more marked, so that in a larva of about 30 mm. in length 

 the right lung extended right back to the cloaca while the left pro- 

 jected back only about 3 mm. behind the glottis. 



In these later stages another important feature is "to be noticed, 

 one which is correlated with the fact that the air-filled lung neces- 

 sarily acts as a float in an aquatic animal. This feature is that the 

 lung tends to assume a position symmetrical about the median plane. 

 Thus in the anterior region where both lungs are present they are 



