f. ■ Nonnentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



worked out we might conclude that the vertebrate teeth were originally organs of the endoderm ! Apart 

 frotn their origin the teeth of Lepidosiren are of interest in comparison with those of Ceratodus in as much 

 as here the primitive condition in which the teeth have not yet fused to form the characteristic dental masses 

 of the adults of existing Dipnoans and which is so beautifully recapitulated in the development of Ceratodus 

 is completely slurred over. 



The tongue is a "primary tongue" exactly similar to that of the embryos of Urodeles though in 

 this case there is no gland field developed in front of its roots as is the case in the Aniphihia. 



The sohd rudiment from which the main part of the buccal cavity is developed by secondary 

 excavation comes in contact with the skin of the ventral surface of the head along a transverse line in front 

 of the outer ends of which the olfactory dimples appear. About stage 34 this area on the ventral side of 

 the head containing the olfactory rudiments becomes enclosed by the lower lip growing forwards while the 

 Upper Hp appears as a ridge enclosing it in front. By the increased development of these lip rudiments 

 the olfactory openings come to be included in the front part of the definitive buccal cavity. 



Visceral clefts. 



Six visceral clefts are laid down in the embryo as solid yolk laden rudiments. The fate of the 

 first (Hyomandibular) has been described by Agar (1906b). It never develops any lumen except at its 

 inner end. It loses its connexion with the skin and, later on, with the pharynx also and eventually its 

 presence is betrayed only by the peculiar Pinkus' organ derived from its outer epiblastic end. 



Clefts II — VI become perforated though eventually in Lepidosiren II becomes closed, leaving the four 

 clefts which persist in the adult. In Lepidosiren the clefts at no time develop regulär respiratory lamellae 

 as in most fishes. Their walls grow out into irregulär rounded respiratory processes which vary greatly 

 in their extent of development in different individuals. 



Lung. (Graham Kerr, 1906, igo8, 1909.) 



The lung arises as a rounded knob about stage 32 projecting downwards in the middle line from 

 the lower side of the still solid pharynx close to its bind end i. e. on a level with cleft VI. The Oesophagus 

 which is already clearly modelled out of the solid mass of yolky endoderm slopes obliquely tailwards, 

 ventralwards and towards the left side. The lung rudiment grows backwards in the median plane, sloping 

 slightly dorsalwards, the Oesophagus being bent well out of its way to the left side. Along the dorsal side 

 of the main mass of yolk a way is as it were prepared for the backwardly growing lung by the formation 

 of a kind of valley along which the lung grows. Later on this valley flattens out and disappears. Already 

 soon after its appearance the lung rudiment becomes bilobed and the two lobes grow backwards as the 

 lungs of the adult. Like so many other organs the lung is quite solid at first and only later on develops 

 a cavity in its interior. 



Thyroid and thymus. 



The thyroid arises from a solid yolk laden rudiment of at first considerable anteroposterior extent 

 which becomes gradually nipped off from the pharynx from behind forwards. The development of the 

 thymus has been described by Bryce (1905) for Lepidosiren. The main thymus buds are derived from the 

 walls of clefts III and IV while abortive buds arise from clefts II and V. 



There is a well marked postbranchial body on the left side developed as a solid projection from the 

 pharyngeal rudiment close to the ventral end of cleft VI. 



