PLATE 2 



EXPLANATION OF FIGURES 



7 to 13 'Type A' or primordial egg cells in various stages of rest and divi- 

 sion. All X 1024. Figures 10 to 13 drawn from same preparation as figure 51. 



7 Primordial egg cell in which the chromatic bodies are very distinct and 

 clearly show duplex nature. From an immature male gonad developed as far 

 as spermatocytes. Length of specimen, 8.5 mm. 



8 Prophase of division of a primordial egg cell. Same preparation as figure 7. 



9 Division of primordial egg cell, after splitting of metaphase chromosomes. 

 From gonad containing spermatogonia and a few spermatocytes. 



10 Anaphase of division of a primordial egg cell, or oogonium. Some ana- 

 phase chromosomes appear double. From specimen 10 mm. long, undergoing 

 period of oogonial division. When collected, specimen was a male. During 

 fifty-seven days it was kept under observation all traces of male character were 

 lost and the shell grew 4 mm. in length (it was originally 6 mm. long). 



11 Telophase of division of a primordial egg cell or oogonium. From same 

 specimen as figure 10. Midbody prominent. 



12 Young daughter elements resulting from division of a primordial egg cell 

 or oogonium. From same specimen as figure 10. 



13 Primordial egg cell or oogonium which has recently been formed by di- 

 vision of a preceding one, with chromatic bodies again becoming evident. From 

 same specimen as figure 10. 



14 to 20 Represent primordial male cells or spermatogonia in various stages 

 of rest and division. All figures drawn from gonads of immature males. X 

 1024. 



14 Primordial sperm cell or spermatogonium. 



15 Prophase of spermatogonial division. 



16 Metaphase of spermatogonial division: 



17 Anaphase of spermatogonial division. 



18 Late anaphase of spermatogonial division. 



19 Telophase of spermatogonial division. 



20 Daughter cells of spermatogonial division. 



21 LTnusual case of an oocyte undergoing partial development in an im- 

 mature male gonad. The animal was quite a large one, however, (11.5 mm.) 

 and it is quite possible that some of the female germ cells had begun to develop 

 before the testis began to form. It will be shown in a separate paper that male 

 development may interrupt and replace early stages of female development. 

 In the cell figured here the synizesis stages have been completed. 



22 Spermatogonial cell recently formed by division, showing prominent 

 mid-body, which may be homologous with granule found at periphery of spermat- 

 oblast. Cells like this and figure 19 suggest that the eccentric position of the 

 nucleus in apyrene spermatoblasts is due to failure to return to a central position 

 after last division. 



23 Primary spermatocyte showing prominent mid-body and spindle fibers 

 remaining from last division. 



56 



