xxvi SIGNIFICANCE OF DEVELOPMENTAL STAGES 305 



longitudinal bands formed by the union of the leaves 

 (M. PI] becoming the muscle-plates, which are thus cell- 

 fusions, each being formed by the union of portions of 

 a series of longitudinally arranged cells. 



At the same time the cells of the splanchnic layer 

 of mesoderm thin out and become the visceral layer of 

 ccelomic epithelium. 



We see then that by the time the larva has reached the 

 stage shown in Fig. 75, it is no longer a mere aggregate of 

 simple cells arranged in certain layers. The cells them- 

 selves have undergone differentiation, some becoming modi- 

 fied into nerve-fibres, others by division and subsequent 

 fusion with their neighbours forming muscle-plates, while 

 others, such as the epithelial cells, remain almost unaltered. 



Thus, in the course of the development of Polygordius, 

 cell-multiplication and cell-differentiation go hand in hand, 

 the result being the formation of those complex tissues the 

 presence of which forms so striking a difference between the 

 worm and the simpler types previously studied. 



It is important to notice that this comparatively complex 

 animal is in one stage of its existence the oosperm as 

 simple as an Amoeba ; in another the polyplast -it is com- 

 parable to a Pandorina or a Volvox ; in a third the 

 gastrula it corresponds in general features with a Hydra ; 

 while in a fourth the trochosphere it resembles in many 

 respects a Medusa. As in other cases we have met with, 

 the comparatively highly-organized form passes through 

 stages in the course of its individual development similar in 

 general characters to those which, on the theory of evolution, 

 its ancestors may be considered to have passed through in 

 their gradual ascent from a lower to a higher stage of 

 organization. 



