302 POLYGORDIUS less. 



the coelome. The cells being arranged in longitudinal series, 

 we have a number of such books placed end to end in 

 a row with the corresponding leaves in contact — page one 

 of the first book being followed by page one of the second, 

 third, fourth, &c., page two by page two, and so on through 

 one or more segments of the trunk. Next, what we have 

 compared with the leaves of the books — the divided 

 portions of the cells — become separated from the backs — 

 the undivided portions (n^) — and each leaf {M. PI) fuses 

 with the corresponding leaves of a certain number of books 

 in the same longitudinal series. The final result is that the 

 undivided portions of the cells (backs of the books, Coel. 

 Epthm) become the parietal layer of ccelomic epithelium, the 

 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. 74, 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 Polygordiijs, 

 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. 



