246 
K. M. SMITH AND H. G. NEWTH. 
ventral extension round the sides of the fore-gut, and, indeed, 
in sections in front of the club-shaped gland the mesoblast 
does not appear at all between the ectoderm and the gut- 
wall (PI. 18, fig*. 1), save for an occasional isolated cell. 
In sections a little further back, in the region where the 
mouth will be formed, the collar somite on either side sends 
a ventral horn downwards round the gut as a thin plate of 
cells ; but since there are no cavities in these extensions and 
the myosepta have not yet assumed their characteristic 
appearance, it is impossible to make out the relations of the 
somites in this stage. 
The right collar cavity is completely separated from the 
gut, but on the left side there is a virtual communication, 
marked by the peculiar orientation of the cells of the gut- 
Avall. 
Our second (and critical) stage is one in which the mouth 
has just become established, but is still a mere pore. PI. 18, 
fig. 2, shows the appearance of a section about 5 /u behind the 
blind anterior end of the gut. The collar somites have con- 
siderable cavities which extend ventrally on either side of 
the pharynx. We will first deal with that of the right side. 
Into its dorsal part projects the mass of the first myotome, 
the cells of which are already differentiated as muscle ; its 
ventral horn can be traced, with diminishing lumen, to the 
mid-ventral line. The next section of the series (PI. 18, 
fig. 3) shows the ventral horn as before ; but in the muscle 
a crescentic septum has appeared, dividing its mass into an 
inner and an outer portion. This septum is the first myo- 
septum, and the inner muscle mass (in . contact with the 
notochord) is the anterior end of the second myotome (i.e , 
first trunk somite). Succeeding sections show the gradual 
increase in size of the trunk somite at the expense of the 
collar somite, as evidenced by the outward and downward 
migration of the septum (PI. 18, figs. 4 and 5). Two sections 
further on (PI. 18, Jig. 6) the cavity of the trunk somite is 
seen to be well established above the dorsal edge of the 
septum, and four sections beyond this the septum has just 
