50 MR W. E. AGAR ON THE DEVELOPMENT OF THE 



forward the loose tissue inside it, so that the latter has accumulated into a thick mass 

 of heavily yolked material in front of the growing point. As if owing to the pressure 

 thus set up, the front end of the notochord ends squarely against the accumulating yolk 

 and other tissue in front of it. Fig. 4, about stage 32, shows the continuation of this 

 process. The secondary or fibrous sheath has been secreted internally to the elastica 

 externa. Finally, in stage 38 (fig. 5), we see an extremely dense mass, representing 

 the massed-up tissue pushed forward by the advancing notochord, enclosed in the 

 extreme front end of the primary sheath. Figs. 2, 3, 4 and 5 are drawn under the 

 same magnification, and show the forward growth of the notochord towards the 

 pituitary body. That the diminishing distance between these two structures is due to 

 the moving forwards of the tip of the notochord, and not only to the backgrowth of 

 the pituitary body, is shown by comparison with other structures, such as the auditory 

 sac and parts of the skull. The growth of the notochord is evidently brought about 

 partly by the vacuolisation of the heavily yolked cellular material occupying the front 

 end (cf. figs. 2, 3, 4, 6 and 5, showing this proceeding pari passu with the absorption of 

 the yolk). At the same time, of course, the whole notochord increases in length by the 

 enlargement of the vacuoles throughout the structure. 



Fig. 6 is drawn under a higher power to show the relations of the sheaths. Both 

 these are fully differentiated before the outer cells of the chorda have arranged them- 

 selves into an epithelium, which takes place about stage 36 + . After the formation of 

 the epithelium the secondary sheath increases very rapidly in thickness, especially its 

 outer layer. Immigration of cartilage cells from the bases of the arcualia begins about 

 stage 36. 



It is interesting to note that the part of the primary sheath in front of the re- 

 advancing tip of the definitive notochord increases, probably in length, and certainly in 

 thickness, though far removed from any epithelial influence. 



The foregoing account applies to Lepidosiren. In Protopterus I was unable to 

 make out the details of the process. The anterior end of the notochord recedes, however, 

 between stages 25 + and 28 + , and then grows forwards again. The whole process 

 takes place much more rapidly in Protopterus than in the other genus, and is practically 

 complete in stage 32. 



The first appearance of the cranium is in stage 31, where the trabecules are repre- 

 sented by concentrations of connective tissue underlying the thalamencephalon and 

 mid -brain ; near their hind ends each is continuous with a downward extension of this 

 tissue, the quadrate. The hyoid arch is also foreshadowed in connective tissue. 



Fig. 7, Plate II., and fig. 13, Plate III., show a reconstruction from horizontal sections 

 of a slightly later stage in Protopterus (about stage 31). This and all the other recon- 

 structions were made by Graham Kerr's method. The only material difference between 

 this and the corresponding stage in Lepidosiren is that in this genus the notochord does 

 not extend so far forward at present. As already mentioned, the re-growth forward of the 

 notochord takes place much more quickly in Protopterus. At this stage the conversion 



