474 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Plate VI. 



Fig. I . Part of section of egg, showing the male pronncleus. The female pronucleus lies nearer 

 the center of the egg, is less regular in outline and has less perinuclear protoplasm. 

 A single polar body (not represented) is seen in this section. It lies close to the sur- 

 face of the egg, beneath the membranes, not far from the male pronucleus. It appears 

 as a small mass of chromatin, which stains quite as intensely as the nuclei. Egg 

 about 6 hours old. x 276. 



Fig. 2. Section of egg with four nuclei, none of which are at the surface, x 152. 



Fig. 3. Part of same section, showing the nucleus and surrounding protoplasm and yolk, x 276. 



Fig. 4. Lateral section, cutting yolk segment on a level with the disk-shaped nucleus. Compare 

 Fig. 5 a. Eight-cell stage. Age about 12 hours. x276. 



Fig. 5. Section through egg in eight-cell stage. Compare Fig. G. Age about 15 hours, x 152. 



Fig. 6. Surface view of egg in the third segmentation or eight-cell stage. The egg membranes 

 have been removed. The nuclei lie at a deeper level than they appear in the draw- 

 ing. Compare Fig. 5. x 78. 



Pig. 7. Section of egg iu the fourth segmentation stage. Sixteen cells, x 152. 



Fig. 8. Fifth segmentation stage. Age, 19 hours. Cells not yet at surface, x 152. 



ITiG. 9. Invagination stage. A solid ingrowth of blastodermic cells has taken place at Ig, where 

 a slight pit is formed. The section cuts obliquely through the invaginate cells, x 152, 



REFERENCE LETTERS. 



a, perinuclear protoplasm. 



Ch, chitinous eijg envelopes (removed, except iu Fig. li). 



Ep, ectoblastic cell. 



Ig, shallow pit of invjigination. 



y.c, yolk spherule. 



