Figures 1-9, 10, 12, 13, 15, 16, 19, 21-24 are views looking doven on egg as 

 floating freely, and, since animal pole is downwards, represent in all cases 

 ventral surface of the blastodisk, embryo, &c., as seen through the transparent 

 intervening yolk-mass. All the sketches are made from the living egg, and 

 changes that might be attributed to death or to efiect of reagents are completely 

 excluded. 



In Plate III. the darker tint indicates blastodisk proper, and the pale tint 

 shading off the intermediary layer. 



In Plate IV. medium tint indicates embryonal shield and structures formed 

 from it, embryo proper being marked by a deeper tint, whilst a light tint washed 

 over whole marks boundary and extension of the growing blastoderm. 



In Plates V. and VI. embryo proper and its parts are alone tinted ; whilst in 

 fig. 27 extent of yolk-mass is shown by a darker tint. 



Plate III. 



Fig. 1. Formation of first furrow in the germinal disk. x26. 



2. Appearance when outline of first two cells is formed. X 26. 



2 a. Transverse section of above, showing that the first furrow penetrates 

 the whole thickness of the germinal disk. X 26. 



3. First formation of the second furrow at right angles to the first. X 26. 



4. Completion of the four-cell stage with nuclei, X 26. 



5. Direction of the furrows which divide the four cells into eight, x 26. 



6. Eight-cell stage after the nuclei have disappeared and the furrows com- 



menced which divide the disk into sixteen cells. x26. 



7. Sixteen-cell stage, complete with nuclei. Outside the disk will be ob- 



served the collection of granules and free nuclei, in which the inter- 

 mediary layer is developed. X 26. 



8. Later segmentation-stage, in which the blastoderm consists of more than 

 one layer of cells, and around the disk will be seen the first row of the 

 intermediary layer cells formed by free cell-formation. X 26. 



0. A little later stage, in which three rows of cells have been formed in the 

 intermediary layer; but the outlines of the cells are not distinguish- 

 able, and the nuclei alone remain to mark the position of each cell. 

 X26. 



