8 
]\IONICA TAYLOE. 
the embryo, and tlien to spread out somewhat so as eventually 
to extend over about half the sphere. This gives the egg a 
very characteristic appearance, the liquid yolk being silvery 
and glistening. 
Stage 19. — By the time the embryo is 2 mm. or a little 
more in length the yolk has been quite covered in. The 
brain is hollow. Tlie rudiments of eyes and ears are assum- 
ing a cavity ; the position of the principal cranial nerves can 
be determined, as well as that of mandibular and hyoid 
arches and the rudiments of the four branchial arches. A 
most careful search, however, has revealed no trace on the 
external surface of gill-slits, either in this or in later stages, 
the slits not becoming perforate till after they are covered 
over by the operculum, so that in this respect Sy mb ranch us 
differs from other Teleosteans. The developing pectoral fins 
are conspicuous in sections at this stage. They contain the 
solid ventral muscle processes of three myotonies of the front 
part of the trunk. The innervation and muscularisation of 
the pectoral fins has been worked out by a study of the 
composition of the brachial plexus, an account of which will 
be given later on. 
No trace of Kupffer’s vesicle can be found in this or in 
earlier stages. 
Stage 21. — The embryonic rudiment has now a ridge-like 
appearance. It curves round the yolk-sac, which has a 
diameter of 2‘9 to 3 mm., through an angle of 170°. By the 
aid of sections the brain is seen to be divided into three 
parts, though the mesencephalon is not nearly so well marked 
off from the prosencephalon, as it is from the rhomben- 
cephalon. The optic vesicles are conspicuous, while the 
mandibular, hyoid and bi-anchial arches with their nerve 
supply are to be seen as an unsegmented, translucent, tripartite 
mass lying on either side of the brain (PI. 1, fig. 5). 'Plie 
fins look like two patches on the surface of the yolk, the 
ventral processes of the myotomes (PI. 1, fig. 5a) like oblong 
blocks spread out on the dorsal part of the yolk-sac. The 
pericardial cavity is beginning to form. PI. 1, fig. 5 gives 
