" GROUSE DISEASE " STRONGYLOSIS 



221 



to segment until the resulting cells are exceedingly small. The mass becomes 



somewhat flattened, and a slight dimple appears at one border of 



the oval disc (Fig. 17). This is the first step towards the formation 



of the cylindrical body of the young embryo. . By the gradual deepen ovum - 



ing of this dimple the egg mass acquires a tadpole - like appearance, the 



anterior end being, thus early, easily distinguished from the posterior end 



of the body. The anterior portion 



soon exhibits a central depression, 



which indicates the commencing for- 



mation of the mouth (Fig. 18). As 



the lateral dimple continues to deepen 



the body mass elongates to such an 



extent as to become folded upon this 



two or three times, in order to become 



accommodated within the shell (Fig. 19). 



The alimentary canal meanwhile has 



gradually been developing, so that by 



the time the embryo attains a cylindrical 



form the canal is found to extend throughout the body as a distinct cell-walled 



tube (Figs. 20, 21). During the whole of this period the embryo remains 



quiescent, but about an hour or so before it leaves the egg-shell it commences 



to exhibit a certain amount of movement. This movement gradually increases 



FlG - 17 - FlG - l8 - 



Developing ova of T. pergracilis. 



FIG. 19. 



FIG. 20. 

 Formation of the larva of T. pergracilis. 



FIG. 21. 



in extent and vigour, until it ultimately overtaxes the resistance of the egg 

 shell, which suddenly ruptures. The success or failure of these efforts on the 



