60 DEAN. 



EXPLANATION OF PLATE IIL 



The Early Development of Acipenser sturio. 



Egg membranes of the stage of Fig. 43 and later, were removed with needles 

 before fixation ; those of the earlier stages were separated in dilute Javelle water. 

 X about 23. 



Fig. 35. Egg immediately prior to fertilization. 



Fig. 36. First cleavage, i hour after fertilization. 



Fig. 37. ij^ hours. 



Fig. 38. Third cleavage, of normal type. 2^ hours. 



Fig. 39. Third cleavage, of meridional type, seen in side view. The second 

 furrow is to be observed traversing the yolk half of the egg. 



Fig. 40. Fourth cleavage, of extremely regular type. 3^ hours. 



Fig. 41. Sixth cleavage. 4 hours. 



Fig. 42. Sixth cleavage. Lower pole of egg. 



Fig. 43. Late segmentation. 6 hours. 



Fig. 44. View of animal pole of stage of 6 hours. 



Fig. 45. Blastula. View of animal pole, showing through its transparent roof 

 the outline of the segmentation cavity. 8^ hours. 



Fig. 46. Blastula in latest stage. 16 hours. 



Fig. 47. Early gastrula. The dorsal lip of the blastopore is seen at the left of 

 the figure. 19^ hours. 



Fig. 48. Gastrula, in side view, dorsal lip at the left. 26 hours. 



Fig. 49. Gastrula, showing faintly marked indentation of the dorsal lip. 28_^ 

 hours. 



Fig. 50. Late gastrula, indicating obscurely the outline of the embryo. 32 

 hours. 



Fig. 51. Embryo of 43 hours. Eight primitive segments are present but are 

 not distinguishable in surface view. In this stage are to be noted : the marked 

 lateral expansion of the head region, the early neurenteric canal, the mode of 

 closure of the blastopore — caused apparently by the more rapid growth of the 

 ventral lip. 



Fig. 52. Embryo of 46 hours: view of head and anterior trunk region. As 

 yet the primitive segments — of which about 14 are now present — are not 

 noticeable in surface view. The brain and optic vesicles are shown in an undif- 

 ferentiated condition, their outlines prominently marked on account of pigmenta- 

 tion. The dark parietal zone of the embryo indicates the limit of the fore-'gut. 

 The region of the pronephric tubules is obscure. 



Fig. 53. Embryo of 46 hours : view of tail region, showing the location of the 

 neurenteric canal and of the almost closed blastopore. 



Fig. 54. Embryo of 48 hours : view of tail region, showing the late appear- 

 ance at the surface of the neurenteric canal. About 20 primitive segments are 

 here present. 



