628 WILSON. 



PLATE XXXIII. 



[Figs. 50-53 from life ; the others from preparations. Figs. 48, 49 x 300; the others x 270.] 



Fig. 48. Early flattened blastula (±512 cells) from the lower pole, showing 

 the last trace of bilaterality. Lettering only for comparison with next figure. 



Fig. 49. The same from the side, in optical section (showing also the lateral 

 cells). 



Fig. 50. Normal gastrula, 7 hours. 



Fig. 51. From shaken 2- and 4-celled stages. One of two separate twins within 

 the same membrane. From the same spawning as the last. 



Fig. 52. Fourth-sized gastrula. With the last. 



Fig. 53. Two }i sized gastrulas. With the last. 



Fig. 54. Fourth-sized gastrula (^/s embryo .''). From shaken 8-celled stage. 8 

 hours. 



Fig. 55. Half -sized gastrula (V4 embryo). From shaken 4-celled stage. 7 

 hours. 



Fig. 56. Half -sized gastrula (Vs embryo.'). From the same preparation as 



Fig- 54- 



Fig. 57. Gastrula intermediate in size between the normal and the % gastrula 

 (cf. Fig. 63 for normal size at this stage). With the last. 



Fig. 58. Fourth-sized gastrula (% embryo). From same preparation as Fig. 55. 



Fig. 59. Unilateral development. Half-sized gastrula, from shaken 2-celled 

 stage, attached to the other undivided ^ blastomere. Membrane intact ; 8 hours. 



Fig. 60. From shaken 2- and 4-celled stages, 7 hours. Attached to a gastrula 

 of rather more than }^ the normal size is a gastrula (?) about yi the normal. 



Figs. 61-62. Two triple gastrulas (membranes intact), 7 hours. From shaken 

 4-celled stage. 



