198 



DEVELOPMENT OF LUNG-FISH 



By this time (Fig. 230) it will be seen that its prominent 

 organs have already been differentiated. There are thus : 

 medullary canal, M, with optic, OP, and auditory, A U, 

 vesicles ; gut with gill slits, GS, neurenteric canal, NC, 

 and suggestion of mouth, 5, and anus, A ; notochord, 

 CH \ segmented mesoderm (primitive segments), PS, 

 and heart, H. The medullary groove was converted into 

 a canal, as has been already suggested, by the overroofing 

 and fusion of the summits of the medullary ridges ; its 

 anterior dilatation is the brain ; the gut, G, communicates 

 freely below with the yolk mass ; it is a cavity, a portion 

 of the coelenteron that has been constricted off with the 

 embryo ; its openings, the mouth, anus, and gill slits, are 

 secondary, acquired after there have been established in 

 these regions fusions of entoderm and ectoderm ; the 

 neurenteric canal, NC, a communication between medul- 

 lary tube and gut, is a structure acquired in the stage of 

 Fig. 226, where the hinder medullary groove was roofed 

 over, allowing, in the region of the tail folds, a communi- 

 cation to exist between medullary canal and coelenteron. 

 The notochord has by this stage been completely sepa- 

 rated from the entoderm ; it already assumes a supporting 

 function. 



III. The Development of Ceratodus 



The development of a Lung-fish has thus far been de- 

 scribed (Semon) only from the outward appearance of the 

 embryo. The egg of Ceratodus (Fig. 192) is seen without 

 its covering membranes, enlarged, in Fig. 231. Its upper 

 pole is distinguished by its fine covering of pigment. The 

 first fine planes of cleavage are shown in Figs. 232-236; 

 and from these it will be seen that the yolk material of the 

 lower pole is not sufficient to prevent the egg's total seg- 



