Normal Plates of the Development of Lepidosiren paradoxa and Protopterus annectens. 5 



Otocyst. 



The otocyst rudiment becomes apparent about stage 20 as a down growth of the deep layer of the 

 ectoderm. A split is apparent almost from the first and this soon widens out to form a rounded cavity. 

 In the subsequent development of the otocyst the chief point of interest seenis to be that the recessus endo- 

 lymphaticus appears to develop in Frotopteras and Lepidosiren as in Ceraiodus (Semon) quite independently 

 of the original connexion of the otocyst with the outer ectoderm. The endolymphatic outgrowth develops 

 considerably mesiad and posterior to the isthmus which forms the last connexion between otocyst and skin. 



Skin sense organs. 



The System of lateral line organs makes its appearance early in development {Lepidosiren, stage 27) 

 as a thickening of the deep layer of the ectoderm on each side anteriorly. This thickening spreads out 

 along the paths marking the adult distribution and becomes divided up into the individual sense organ 

 rudiments. These are at first arranged segmentally in the trunk region. In Lepidosiren the organs retain 

 their primitive superficial position while in Protopterus they become in the head region eventually sunk 

 beneath the surface in tubulär Channels. 



PINKUS' organ. 



Agar (1906b) has described how the ectodermal outer end of the spiracular cleft becomes converted 

 into the sensory organ of Pinkus. 



Pituitary body. (Graham Kerr, 1901 d.) 

 The development of the pituitary body takes place after the normal Amphibian manner. Arising 

 as a wedge shaped ingrowth from the ectoderm about stage 23, the inner end becomes enlarged and develops 

 a cavity as a secondary split. About the same time as the split appears the organ loses its connexion 

 with the skin through its undilated portion undergoing atrophy. 



Alimentary canal. (Graham Kerr, 1909.) 

 The differentiation of the alimentary canal out of the primitive mass of yolk may be said to begin 

 about stage 23 with the modelling of its anterior region caused by the precocious development of meso- 

 dermal tissues which foreshadows the development of heart and pericardium. In this way the region of 

 the foregut becomes marked off from the midgut which serves as the great storehouse of yolk and which 

 no doubt on this account is much retarded in its differentiation. 



Buccal cavity. 



The main part of the buccal cavity is developed as a secondary excavation in the originally solid 

 yolkladen anterior part of the enteric rudiment developed from the macromeres. The outer part of the 

 buccal rudiment becomes converted into the epithelial lining of the buccal cavity. Examination of celloidin 

 sections shows that there is no actual ingrowth of ectoderm along the surface of the buccal rudiment. This 

 mode of development of the buccal lining is no doubt secondary but it is of interest as emphasizing the 

 possibility of grave error in using embryological evidence for deciding as to the morphological nature of 

 Organs which develop in proximity to the boundary region between two germinal layers. Were the two 

 Dipnoans now under discussion and certain Urodeles the only vertebrates whose embryology had been 



