The life history of Diplodiscus temporatiis Stafford. 659 



Plate 33. 



Fig. 42. Egg in resting stage before the first segmentation division. 



Fig. 43. Section, oblique to plane of equatorial plate, through egg 

 at metaphase of the first segmentation division. 



Fig. 44. Side view of an egg at the metaphase of the first segmen- 

 tation division. 



Fig, 45. Section through equatorial plate of egg in first cleavage. 



Fig. 46. Two cell stage, following first cleavage. 



Fig. 47, Three cell stage, result of second cleavage. 



Fig, 48. A four cell stage, resulting from irregular segmentation of 

 the micromeres. 



Fig. 49. Five cell stage : the usual result of the third cleavage. 



Fig. 50. Section through an embryo showing the formation of the 

 investing membrane. 



Fig. 51. Slightly older stage, showing the characteristic degenerative 

 changes in the nuclei of the investing cells. 



Fig. 52. Section through an embryo in which all of the cells, with 

 the exception of the endoderm cells, have lost their outlines. 



Fig. 53. Section through an embryo in a later stage of development ; 

 the endoderm is clearly marked off from the ectoderm. 



Fig. 54. Section through an embryo in which there is a well defined 

 layer of cells between the ectoderm and the endoderm. 



Fig. 55. Section through an embryo in which there is no distinction 

 between the cell layers, except for the greater size of the endoderm nuclei. 



Fig. 56. Longitudinal section through an embryo showing the pri- 

 mordia of the gut, of the anterior sucking disc and pharynx, and of the 

 water vascular system. 



