EXPLANATION OF PLATE XlY—rondwml, 



Fig. 75. — Tlie cleavage has begun. The leiigtii of the anijjhiuster has 

 readied its maximum. 



Fig. 76. — First cleavage completed, but the two nuclei are still connected 

 by a slender band of nucleoplasm. 



Fig. 76 R. — One of the two nuclei in fiir. 62, magnified 160 diam. 



Fig. 77- — A section of the upper polar ring, taken about 1 mm. under 

 the surface. The more darkly shaded ring around the base of the calotte 

 is seldom seen. Magnified 50 diam. 



Fig. 78. — One pole of the ampliiaster (fig. 73), magnified 440 diam. 



Fig. 79. — A transverse section through the middle of an egg arrived at 

 stage 26, PL XI I. T represents the nucleus of one of the ecloblasts at 

 about the same date, magnified 440 diam. 



Fig. so. — A horizontal section of stage 36, PI. XII, taken just below the 

 germinal disc. 



Fig. 81. — A transverse section of stage 37. PI. XII, taken just behind 

 the germinal disc, to show tiie positions of the mesoblasts (x and xy). Cells 

 are seen leading from the nuclear poles of the mesoblasts up to the germi- 

 nal disc. 



Fig. 82. — A longitudinal section of the same date, which passes through 

 tiie middle of the left mesobhisi (x). 



Fig. S3. — A horizontal section of the same date, below the germinal disc. 

 The nuclei lie near the surface of 1 he blastomeres. 



Fig. 84. — A transverse section just in front of the neuroblasts (stage 41, 

 PI. XII). A line of cells leads from each mesoblast to the germ-band of 

 the corresponding side. 



Fig. 85. — A transverse section at a little later date. 



Fig. 86.- — A median transverse section (stage 44, PI. XII). r and x^ 

 have changed their positions with reference to each other in consequence of 

 the invagination. Compare with fisr. 84. 



Fig. 87. — A horizontal section (stage 43, PI. XII). In the anterior part 

 (lower part of the figure) the germ-bands have united, but in the posterior 

 part they are still unclosed. 



Fig. 88. — A median transverse section at the time of hatching, showing 

 the nerve-cells (w) and a pair of segment-cells (s). Magnified 160 diam. 



Fig. 89. — A similar section two days later. 



PLATE XV. 



Fig. 90. — A sagittal section, showing the origin of the nerve-cells and 

 the segment-cells. Just hatched. Zeiss. A, A. and camera. 



Fig. 91. — Embryo freed from the yolk, showing the number of body- 

 segments and the position of the segment-cells (s). (2| days old.) Mag- 

 nified 50 diam. 



Fig. 92. — A segmental organ and one of the segment-cells 3| days after 

 exclusion. Magnified 440 diam. 



Fig. 93. — A sagittal section (constructed from two successive sections) 

 showing the condition of the entoderm eight days after exclusion. Magni- 

 fied about 25 diam. 



Fig. 94. — A portion of a horizontal section, one day later. The ento- 

 (leniiie cills are arranged in a single compact layer. Magnified 160 diam. 



Fig. 95. — A portion of fig. 93 magnified 160 diam. The entodermic 

 cells are loosely arranged in the periphery of the yolk. 



Fig. 96. — A horizontal section, passing near the middle of the pharynx. 

 About three days old. Magnified 50 diam. 



