THE ENTEROPNEUSTA. 21 



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 39), 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 growing with the growth of the animal, and that their 



