160 Gastrulation and Embryo Formation in Amia Calva 
Fic. 10. Embryo about seventy hours after fertilization, embryo longer, 
neural (?) trench extended, blastopore closing. 
Figs. 11, 12. Anterior and posterior portions of embryo about seventy-five 
hours after fertilization, viewed from above, optic thickenings visible, two or 
three protovertebrae differentiated. 
Figs. 13, 14. Anterior and posterior portions of embryo, eighty hours after 
fertilization, viewed from above, divisions of brain and precerebral process 
apparent, seven to nine protovertebrae, beginnings of pronephric ducts, extent 
of mesoblast indicated by dark area around head. 
Fies. 15, 16. Anterior and posterior portions of embryo about ninety hours 
after fertilization, viewed from above, showing pre-cerebral process, optic 
thickenings, first gill arch and cleft, about sixteen protovertebrae, dark 
shading around embryo indicates extent of coelom. 
Fic. 17. Anterior portion of embryo about one hundred and five hours after 
fertilization, viewed from above, showing divisions of brain, optic vesicles, 
adhesive organs, median knob, two gill clefts, beginnings of auditory vesicles. 
Fic. 18. Anterior portion of embryo about one hundred and ten hours after 
fertilization, showing slight advance beyond condition represented in Fig. 17. 
Fics. 19, 20. Anterior and posterior portions of embryo about one hundred 
and twenty-five hours after fertilization, viewed from above, showing, in ad- 
dition to structures previously described, the anlage of the heart, the begin- 
nings of the olfactory organs, neuromeres, three gill arches and their cor- 
responding clefts. 
PEATE Ue 
Fic. 21. Meridional section of an egg about nine hours after fertilization. 
Showing early condition of blastodisc segmentation cavity and yolk. 
Fic. 22. Meridional section of egg about twelve hours after fertilization. 
Showing the yolk masses actively budding off entoblastic cells, and obscuring 
more or less the segmentation cavity. 
Fie. 23.. Meridional section of egg about forty hours after fertilization 
showing proliferation of superficial ectoblast at the point where invagination 
begins. 
Fic. 24. Meridional section of egg. about fifty hours after fertilization 
showing beginning of invagination and maximal thickness of blastodisc. 
Fic. 25. Portion of meridional section of egg of about same age as above 
through region of blastopore showing first formation of mes-entoblast. 
Fic. 26. Meridional section of egg slightly older than preceding stage show- 
ing further extension of gastral cavity. 
Fic. 27. Meridional section of an egg about fifty-three hours after fertili- 
zation, showing reduction in number of entoblastic cells, also thinning of 
blastodisc. 
Fic. 28. Portion of meridional section of egg in same stage as above more 
highly magnified, showing region of blastopore. 
Fic. 29. Meridional section of an egg in stage somewhat later than above 
showing further thickening of margin of blastodisc. 
Fic. 30. Horizontal section of egg just above level of equator, showing 
lateral extent of embryonic anlage, also beginning of notochord. 
