158 John H. Geroüld, 



accomplished in quadrants C and I), Ic^-^-^ and Id^'^'^ being rosette cells. 

 The "intermediate" cells are being formed by the division of quartet 

 la^'^ — 7f/^*-, spindles being seen in Ib^-^ and 7c^"^ Compare with Text 

 Fig. C, which shows the completed rosette in this same egg. 



Fig. 36. D quadrant in 8 — 16-cell stage. 



Fig. 37 — 39. Development of D quadrant, and formation of 4d, 

 drawn from a single egg. 



Plate 6. 



Drawings of actual sections of embryos of Ph. vulgare with the ex- 

 ception of Fig. 41, which is a surface view. The magnification of each 

 figure is 790 : 1. 



Fig. 40a. Parasagittal section of an embryo fourteen and a half 

 hours old, sho\nng the blastopore, somatic plate and dorsal cord (in the 

 interruption of the prototroch). The nucleus of the pole cell of the meso- 

 derm, lying in the adjacent section, is drawn in dotted lines. Three 

 coelomesoblast cells and the two small entomesoblast cells, which are 

 connected with the pole cell, are shown. 



Fig. 40b. Sagittal section of the same embryo, showing no mesoderm. 

 A large dorsal endoderm cell, flattened laterally, separates the two meso- 

 derm bands, one of which is shown in Fig. 40a. 



Fig. 41. Surface view of the somatic plate of an embryo fourteen 

 and a half hours old, killed with Peeenyi's fluid and mounted unstained 

 in glycerine. This shows, somewhat diagramraatically, the principal cells 

 and the bilateral cleavage of the somatic plate. The mesoderm cells ot 

 the left side are drawn in dotted lines. 



Fig. 42. Frontal section immediately dorsal to the blastophore of 

 an embryo fourteen and a half hours old, showing the three more dorsal 

 mesoderm cells of each side and, in dotted lines, the pole cells, which lie 

 at a lower level. The larger cells of the somatic plate are undergoing 

 bilateral cleavage. 



Fig. 43. Obliquely sagittal section of an embryo twenty-four hours 

 old, showing the establishment of the stomadaeum, a mesoderm band, pro- 

 totroch, somatic plate, etc. 



Fig. 44. Part of a longitudinal section of an embryo twenty-four 

 hours old, showing the origin of a (ventral) retractor muscle from cells 

 of the apical and somatic plates, adjacent to the prototroch. 



Fig. 45. Cross section of an embryo twenty four hours old, showing 

 prototroch, mesoderm bands already split into somatopleure and splanchno- 

 pleure, endoderm, and dorsal cord of ectoderm. 



Plate 7. 



Trochophores and larvae of PJk nihjare, drawn from living specimens, 

 except Fig. 47, 48 and 49. 



