56 DEAN. 



EXPLANATION OF PLATE L 



The Early Development of Lepidosteus. 



All figures have been drawn from material fixed in alcoholic picro-sulphuric 

 acid : the egg membranes of the early stages (Figs. 1-7), not separated readily by 

 needles, have been removed after a brief immersion in 20% Javelle water. 

 X about 20. 



Fig. I . Egg immediately before the appearance of the first cleavage furrow. 

 ^^ hour after fertilization. 



Fig. 2. First cleavage: it is noted as a trench-like furrow, extending no 

 further marginally than the limit of the germ disc, i hour. 



Fig. 3. Second cleavage : limited as before to the germ disc, whose margins 

 now merge slightly into the yolk mass. 2 hours. 



Fig. 4. Third cleavage : similar in marginal extension to first and second. 

 3 hours. 



Fig. 5. Third cleavage : seen in side view. The margin of the germ disc is 

 indicated. 



Fig. 6. Fourth cleavage : similar in its limits to the third cleavage. t,% hours. 



Fig. 7. Fifth cleavage. 4jt^ hours. 



Fig. 8. Sixth cleavage. Horizontal cell division occurs for the first time: it 

 is to be noted irregularly within the ring of marginal cells. 5^ hours. 



Fig. 9. Seventh cleavage. Meridional furrows, which in the preceding stage 

 have attained almost the equatorial region of the egg, appear in this stage greatly 

 reduced. 6% hours. 



Fig. 10. Eighth (?) cleavage. In this stage an irregular marginal depression 

 and an elevated mass of cells of the animal pole are to be observed. 8 hours. 



Fig. II. Late segmentation, showing marginal groove and flattened cell cap. 

 20 hours. 



Fig. 12. Late segmentation. The thickened cell cap is shown in a stage in 

 which it is coming to overspread the marginal groove. 25 hours. 



Fig. 13. Early gastrulation. 32 hours. 



Fig. 14. Gastrulation. In front of the blastoderm's marginal indentation the 

 embryonic thickening is to be faintly seen. 38 hours. 



Fig. 15. Late gastrulation, showing circular blastopore. 44 hours. 



Fig. 16. Late gastrulation, a stage slightly older than that of Fig. 15. An in- 

 dented blastopore rim is here exceptionally present. 



Fig. 17. Early embryo, showing the flattened vascular area in front of the 

 head eminence, and the closed blastopore. 60 hours. 



Fig. 18. Side view of the embryo of 60 hours. 



Fig. 19. Embryo, showing an early stage of the establishment of its external 

 form. 80 hours. 



Fig. 20. Embryo of 90 hours. 



