PLATE 8. 



Fig. 23. Horizontal section of somewhat older larva than that repre- 

 sented in fig. 22. Here further irregularity appears among cells at anterior 

 of archenteric wall. X 190. 



Fig. 24. Horizontal section of larva. Here the irregular disposition of 

 cells indicated in fig. 23 is still shown but with a tendency to grow back- 

 ward between the ectoderm and endoderm or archenteric wall. This wedge 

 of cells is the anlage of the mesoderm. X 190. 



Fig. 25. Horizontal section of larva. Here the wedge of cells has 

 reached beyond the collar region. X 190. 



Fig. 26. Horizontal section of larva showing formation of second body 

 cavity. X 190. 



Fig. 27. Transverse section of larva somewhat older than that figured 

 in 26. Section cuts across second body cavity. X 190. 



Fig. 28. Horizontal section of larva showing formation of third body 

 cavity, and the relation of the three body cavities to one another. X 190. 



Fig. 29. Part of similar section of same larva figured in 28 but more 

 enlarged. This figure shows communication between cavities 1 and 2, and 

 3. X 300. 



[226] 



