446 Julia B. Platt 
in exactly the same way as the next following protovertebra of the 
trunk. Now, according to v. Wine (82), and both Neat (’97) and 
HOFFMANN (97) confirm the statement, the fifth somite does not 
develop muscular tissue in the Selachii, being crushed out of existence 
by the growth of the auditory vesicle, and the muscle-plates begin 
with the sixth somite, which lies opposite the vagus Anlage. 
One is therefore surprised when RAgBL adds (l. c. page 236), 
»while foregoing a criticism of the recent hypotheses regarding the 
metamerism of the Vertebrate head; ich bemerke nur, dass sie mir 
fast sämmtlich den Eindruck einer Beweisführung um jeden Preis, 
selbst um den Preis der Thatsachen, gemacht haben«. 
It appears to have suited Ragr’s theory that the muscle-plates 
should begin with the fifth somite, and should develop from before 
backwards. Do we, who have other theories, not deserve charitable 
criticism — even from RABL? 
Leaving for the time the question of the metameric value of the 
segments of the head, I now describe the primitive relations of the 
postotie divisions of the axial mesoderm in Necturus. In fig. 20, 
pl. XVIII, a sagittal section is shown through the first three postotic 
somites, corresponding to v. WisHE’s 5%, 6 and 7" somites, as in- 
dicated by the figures in parentheses. At the right of the section 
the wall of the alimentary canal is seen, while at the left the section 
is bounded by ectoderm which deepens anteriorly in the invagination 
of the auditory epithelium. The Anlagen of three ganglia are sectioned, 
those of the facial, glossopharyngeal, and vagus nerves. A group 
of scattered cells partly below the auditory epithelium, partly below 
the glossopharyngeal ganglion represents the first postotic somite. 
The mesoderm of the second postotic somite rises higher than that 
of the first, but is also interrupted by the ganglionic cells above, 
while the third postotic somite alone reaches the usual height of the 
protovertebra. It is noticed that the position of the glossopharyngeal 
and vagus ganglia is such that these ganglia complete dorsally the 
ventral divisions of the mesoderm above which they lie, giving to 
the section the semblance of three similar divisions of tissue lying 
between the endoderm and external ectoderm. 
The sections represented in figs. 21 and 22, are from a slightly 
older embryo than that from which the section seen in fig. 20 is 
taken, and are reversed in mounting. Fig. 22 shows a section median 
to that shown in fig. 21. The ganglionic Anlagen are not as large 
relatively as in fig. 20, since the cells have migrated outwards. In 
