58 DEAN. 



EXPLANATION OF PLATE IL 



The Early Development of Lepidosteus. 



Fig. 21. Vertical section of egg of about i hour after fertilization, showing the 

 first division of the nucleus. X 20. 



Fig. 22. Vertical section of stage of 2 blastomeres. X 20. 



Fig. 23. Horizontal section through the nuclei of 4-cell stage. X 20. 



Fig. 24. Similar section of 8-cell stage. X 25. 



Fig. 25. Similar section of i6-cell stage. X 25. 



Fig. 26. Vertical section of i6-cell stage. X 20. 



Fig. 27. Vertical section of stage of sixth cleavage, showing yolk nuclei, m. 

 X 20. 



Fig. 28. Vertical section of stage of 20 hours, showing zone of yolk nuclei. 

 X about 55. 



Fig. 29. Vertical section of stage of 25 hours. X about 65. 



Fig. 30. Vertical (sagittal) section of stage of 32 hours. X about 45. 



Fig. 31. Vertical (sagittal) section of stage of 40 hours. Coelenteron, c ; dor- 

 sal lip of blastopore, d ; segmentation cavity, s ; innermost limit of coelenteron, 

 X, X. X 30. 



Fig. 32. Vertical (sagittal) section of region of blastopore in stage of 42 

 hours. Coelenteron, c ; dorsal lip of blastopore, dl; epidermic stratum of outer 

 germ layer, e\ inner germ layer, i; middle, m ; outer, o\ yolk, j. X about 100. 



EiG. 33. Transverse section of hinder trunk region of embryo of 80 hours 

 (.= 7 primitive segments). Chorda, c ; epidermic stratum of outer germ layer, e\ 

 middle germ layer, m, inner, i ; neuron, n. 



Fig. 34. Sagittal section of embryo of 44 hours. Location of closed blasto- 

 pore, bp ; epidermic stratum of outer germ layer, e ; inner layer, / ; middle layer, 

 m ; outer layer, o ; here of head region ; yolk, y. X 75. 



The material of the above sections was fixed in alcoholic picro-sulphuric acid ; 

 Figs. 21, 22, 23, 35, Delafield's haematoxylin, 24-29, concentrated alcoholic picric 

 and acid fuchsin, the remainder, haemacalcium. Egg membranes when present in 

 preparations have not been represented. With few exceptions the figures are 

 from camera drawings- 



