474 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PLATE VI. 



FIG. 1. Part of section of egg, showing the male pronucleus. The female pronucleus lies nearer 

 the center of the egg, is less regular in outline uid 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!52. 



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 witli 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. 6. Age about 15 hours. x!52. 



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. 



FIG. 7. Section of egg in the fourth segmentation stage. Sixteen cells. x!52. 



FIG. 8. Fifth segmentation stage. Age, 19 hours. Cells not yet at surface. x!52. 



FIG. 9. Imagination stage. A solid ingrowth of blastodermic cells has taken place at ly, where 

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



REFERENCE LETTERS. 



a, perinnclear protoplasm. 



Ch, chitinous egg envelopes (removed, except in Fig. r>). 



Ep, ectoblastic cell. 



Ig, shallow pit of invagiuation. 



y. c., yolk spherule. 



