70. 



Fig. 25. Four col led. stage. 



Fig. 26. Eigiit celled stage seen from the animal pole. 



Fig. 27. Eiglit celled stage. The cells prepax'ing for 

 tlae next o 1 ea^'-age . 



Figs. 28 and 29. Later cleavage stages. 



Fig. 30. Ciliated embrvo about 20 hoars old. 



Fig. 31. Extrenal appearance of an ernbryo abo^at 

 hours old. 



Fig. 32. External appearance of an embrvo about 

 hours old. 



Fig. 33. Reconstruction of an .embryo about 105 hours old. 

 The outli2ie and test cells were obtained from a 

 whole mount. The cilia are indicated only at 

 the margins. 



Fig. 34. Reconstruction of an. embryo that has just cast 

 the test, represented with the right cell valve 

 and mantle lobe removed. The stomodaeum and 

 the stalk extending from the cerebral ganglia, 

 together with adhering test cells, are still at- 

 tached. 



Fig. 35. Reconstruction of an embryo about 10 hours after 

 casting. Represented mth the left shell valve 

 and mantle lobe removed. 



