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, 1, with optic, OP, and auditory, AU, 
vesicles; gut with gill slits, GS, neurenteric canal, VC, 
and suggestion of mouth, S, 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, VC, 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 ccelenteron. 
The notochord has by this stage been completely sepa- | 
rated from the entoderm; it already assumes a supporting | 
function. 
III. Zhe 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- 
