72 Papers from the Marine Biological Laboratory at Tortugas. 



FIG. 71. Prophase of second polar mitosis. Several of the chromosomes are arranged 

 radially on the spindle. Residual substance still persists. X 2100. 



FIG. 72. Metaphase of second polar mitosis. The bilobed products of the premature 

 fission of the anaphase of the first mitosis are simply drawn apart to 

 opposite poles. X 2100. 



FIG. 73. Late anaphase of the second polar mitosis. Many chromosomes still bilobed. 

 Two chromosomes are seen to lag far behind. Mid-body again very con- 

 spicuous. < 2100. 



FIG. 74. Telophase of second mitosis. Second polar body constricted off with mostly 

 bilobed chromosomes. Those remaining in egg also bilobed or globular. 

 The arrangement of the chromosomes in the first polar body indicates an 

 abortive attempt at a division. X 2100. 



FIG. 75. Female pronucleus forming as a vesicle containing the chromosomes, and 

 surrounded by rays of central pole of second maturation spindle. X 2100. 



PLATE 6. 



FIGS. 76, 77. Cross-sections of prophase figures of first maturation mitosis with 18 

 chromosomes. X 1320. 



FIG. 78. Two consecutive sections of a metaphase figure of first maturation spindle. 

 Note size differences among the chromosomes. X 2100. 



FIG. 79. Cross-section of prophase figure of first maturation spindle with 20 chro- 

 mosomes. X 2100. 



FIG. 80. Cross-section of distal pole of first maturation spindle at anaphase showing 

 18 chromosomes. X 1320. 



FIG. Si. Female pronucleus with chromatin nucleolus and stout deep-staining network. 

 X 1320. 



FIGS. 82, 83, 84. Different types of female promiclei, with plasmosomes showing varia- 

 tion in chromatin distribution over the linin network. X 1320. 



FIG. 85. Fertilized ovum showing the male and female pronuclei, both with plastin 

 nucleoli (plasmosomes). X 700. 



FIG. 86. Section through the pronuclei in process of fusion to form the segmentation 

 nucleus. Nucleoli have disappeared. X 2100. 



PLATE 7. 

 Hipponoe esculenta. 



FIG. 87. Two oocytes near the culmination of the growth-period, each with several 

 chromatin nucleoli and but slightly chromatic nuclear reticulum. X 440. 



FIG. 88. Primary oocyte at slightly later stage ; nuclear reticulum more highly chro- 

 matic. < 440. 



FIGS. 89, 90. Primary oocytes at still later stages of development, showing but a single 

 nucleolus and stout, compact, and very chromatic nuclear network. X 440. 



FIG. 91. Oocyte at beginning of maturation. Nuclear wall is disappearing. The 

 nuclear reticulum in form of compact spireme. First polar spindle in 

 process of origin. Nucleolus has vanished. X 440. 



FIG. 92. Early prophase of first maturation division. Chromosomes still in form 

 of loose, stout spireme and surrounded by residual substance of nucleus. 

 X440. 



FIG. 93. First polar spindle at early metaphase. X 2100. 



FIG. 94. Second polar spindle at anaphase. X 2100. 



FIG. 05. Mature ovum with female pronucleus. X 440. 



