DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 101 
alternatives, that the growth occurs at the apex or at any point 
intermediate between the two ends is unlikely, from the fact 
that almost immediately after two gill-slits the tissue of which 
it is composed becomes vacuolated and irregular, undergoing 
the “degeneration” characteristic of notochordal substance, 
presenting therefore by no means the appearance of a growing 
tissue. 
The length and proportions of the notochord at this stage are 
indicated in fig. 45, which is, however, not a truly median 
section. Its anterior end already projects far into the anterior 
body cavity, pushing in the mesoblastic lining. Fig. 45 is from 
a specimen slightly older than that from which figs. 37, &c., 
are taken, and the commencing degeneration of the notochord 
tisssue is already begun. 
To recapitulate: the growth of the notochord is due to: 
1. A forward growth of the dorsal anterior portion of the 
archenteron (fig. 30). This is supplemented by— 
2. A longitudinal constriction of the dorsal region of the 
pharynx, which gradually travels backwards (cp. figs. 21 and 22 
with figs. 38 and 389), separating a hollow hypoblastic tube 
which remains open to the gut behind. 
3. A forward growth from the point of junction with the gut. 
In connection with the notochord must be mentioned the 
skeletal rods, which now just appear, though it cannot be 
positively affirmed that they are of hypoblastic origin. When 
first visible, they are two short rods of a deeply-stained, struc- 
tureless substance, which lie in the angles between the noto- 
chord and the dorsal wall of the pharynx. As first seen in this 
position they appear to be formed externally to the hypoblast 
cells, against the ends of which they lie. Their posterior ends 
are enclosed in the hypoblast (fig. 39), and it is difficult to 
understand how this can have been brought about if they were 
secreted by the mesoblast cells. This would involve an outward 
growth of the hypoblast to inclose them, of which there is no 
appearance. As will afterwards be seen the view that they are 
of hypoblastic origin is supported by the fact that they con- 
tinue growiug with the growth of the animal, and that their 
