36 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Fig. 23. Inner and outer ends of the left excretory organ and genital duct, seen 

 from the ventral side. 



Fig. 24. Two celled stage. 

 Fig. 25. Four celled stage. 



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

 Fig. 27. Eight celled stage. The cells preparing for the next cleavage. 

 Figs. 28 and 29. Later cleavage stages. 

 Fig. 30. Ciliated embryo about 14 hours old. 

 Fig. 31. External appearance of an embryo about 22 hours old. 

 Fig. 32. External appearance of an embryo about 70 hours old. 

 Fig. 33. Reconstruction of an embryo about 90 hours old. The outline and test- 

 cells were obtained from a whole mount. The cilia are indicated only at the 

 margins. The anus breaks through later. 



Fig. 34. Reconstruction of an embryo that has just cast the test, represented 

 with the right shell-valve and mantle-lobe removed. The stomodfeum and the stalk 

 extending from the cerebral ganglia, together with adhering test-cells, are still 

 attached. 



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

 with the left shell-valve and mantle-lobe removed. 



Fig. 36. Reconstruction of an embryo about 12 days old. Represented with the 

 right shell-valve and mantle-lobe removed. 



Fig. 37. Section of an embryo about 10 hours old. The plane of the section lies 

 through the blastopore. 



Figs. 38, 39 and 40. Successive sections of an embryo about 13 hours old. Some 

 of the surface cells are beginning to assume the character of test-cells. The group 

 of cells eg., figures 38 and 39, are connected with the cells ap., figure 40, by cells of 

 the same character. 



Fig. 41. Median vertical section of an embryo about 17 hours old. 

 Fig. 42. Median vertical section of an embryo about 22 hours old. 

 Fig. 43. Median vertical section of an embryo about 30 hours old. 

 Fig. 44. Median vertical section of an embryo about 36 hours old. 

 Fig. 45. Transverse section of an embryo about 35 hours old, taken near the 

 third band of cilia. 



Fig. 46. Transverse section of an embryo about 45 hours old, taken through 

 the cerebral pouches. 



Fig. 47. A section similar to the preceding showing the pouches separate. 

 Fig. 48. Transverse section of an embryo about 45 hours old, taken where the 

 stomodseum joins the mid-gut. 



Fig. 49. Transverse section of the same embryo as the preceding, a little further 

 posterior. 



Fig. 50. Section of the gill rudiment in the plane indicated by the line 50, 

 Fig. 36. 



Fig. 51. Section of the same gill rudiment as the preceding in the plane indi- 

 cated by the line 51, Fig. 36. 



Figs. 52, 53 and 54. Transverse sections showing the formation of the " sole" of 

 the foot. 



