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 Deveiop- 

 the oval disc (Fig. 17). This is the first step towards the formation "han^fes in 

 of the cylindrical body of the young embryo. By the gradual deepen °'^""'"- 

 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 

 thetimethe 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 



Fig. 17. Fig. 18. 



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 eff"orts on the 



