258 
by several small cavities or breaks in the tissue, which finally fuse 
with one another and with the paired praemandibular cavities to form 
the single praemandibular cavity of later embryonic life. There can 
be no question here of a line of somites recurved upon itself such as 
Dourn !) describes in Torpedo, for the alimentary canal, from which 
alone such somites could be formed in the anterior region of the head 
(beyond the limit of the mesodermic plates), extends forward of the 
infundibulum, and its anterior extremity is not recurved by the cranial 
flexure, but cut off. 
The position of the cells proliferated from this severed portion 
of the canal, to form the walls of the anterior pair of head cavities 
is seen from the side in fig. 9 a. The cells which are to form the 
walls of the praemandibular cavity are seen at 7. The mandibular 
cavity, which lies, in its anterior part, lateral to the praemandibular, 
has not been represented to avoid confusion. In fig. 9, the two anterior 
pairs of cavities have not as yet appeared in the mesoderm that is 
to form their walls. Fig. 10 is a section showing the relative positions 
of the anterior cavity (a), the praemandibular (7) and the mandi- 
bular (2) after the appearance of the two anterior pairs of cavities, 
and when the constantly increasing cranial flexure has pushed cavity 
a dorsalwards, while crowding the praemandibular cavity still further 
below the mandibular. I leave these head cavities here, since my 
former paper (loc. cit.) continues their development beyond this stage. 
Fig. 9. Diagram showing from the side the relation of the mesoderm in which the 
, anterior’ and the praemandibular cavities are to appear. ect ectoderm. dr brain. 
Ch chorda. 1 praemandibular Anlage. m fusion of 1 with the ectoderm. inf infundi- 
bulum. a Anlage of the anterior head cavity, x fusion of a with the ectoderm. opt 
optic vesicle. 
Fig. 10. Sagittal section showing the relation of the three anterior head cavitiesIto 
one another. ect ectoderm. V trigeminus. c. g. ciliary ganglion. dr brain. opt optic 
stalk. a anterior head cavity. 1 praemandibular cavity. 2 mandibular cavity. 
1) A. Dosen, „Neue Grundlagen zur Beurteilung der Metamerie des 
Kopfes“. Mitteilungen a. d. Zool. Station z. Neapel, 1890. 
