ILLUSTRATIONS. 



Plate 1. All of the figures on this plate were drawn with a Zeiss compensating ocular 8 

 and 2-mm. immersion objective, giving a magnification, when drawn with a 

 camera lucida on an inclined drawing-board slightly above table level, of about 

 1,800 diameters. These figures have laeen reduced one-half in reproduction. 



Fig. a, Cidaris X Cidaris. Anaphase first cleavage. 



Fig. b, Cidaris X Cidaris. Later anaphase first cleavage. 



Fig. c, Cidaris 9 X Tripne^istes d' . Telophase first cleavage. Zwischenkorper at 

 line of contact between cells. 



Fig. d, Cidaris 9 X Tripneustes d' . Late telophase second cleavage. 



Fig. e, Cidaris 9 X Tripneustes cp . Earlier telophase first cleavage. 



Plate 2. Magnification and reduction as in plate 1. 



Fig. a, Cidaris 9 X Tripneustes (p . Anaphase second cleavage; lagging chromosomes at 

 center. 



Fig. b, Cidaris 9 X TripneiistescP. Second cleavage completed; Zwischenkorper. 



Fig. c, Cidaris X Cidaris. Prophase of mitosis; fat droplets small and have been 

 moved out from region of nucleus. 



Fig. d, Cidaris X Cidaris. Late prophase of mitosis; region of asters freed from fat 

 droplets. 



Fig. e, Cidaris 9 X Lytechinus cP . Large fat droplets surrounding nucleus. 



Fig. f, CidarisX Cidaris. Late telophase; chromosomal vesicles; small fat droplets 

 moving in. 



Fig. g, Cidaris X Cidaris. Chromosomal vesicle; large fat droplets. 



Figs, h to l, Cidaris X Cidaris. Small area of cytoplasm from one side of the amphi- 

 aster during anaphase of mitosis, showing variation in size and relative abun- 

 dance of droplets. 



Fig. h, Clear area surrounding each droplet. 



Fig. I, Cytoplasm with a few large droplets. 



Fig. j, Cytoplasm with both larger and smaller droplets. 



Fig. k, Cytoplasm with numerous small droplets only. 



Fig. l, Cytoplasm with numerous large droplets. 



Plate 3. In all of the detailed figures on this plate only the archenteron and a small 

 portion of the posterior end of the gastrula are shown. The relation of the part 

 shown to the entire gastrula may be seen by reference to figure j. All figures 

 except figure j were drawn at a magnification of 1,900 diameters. They have 

 been reduced one-half in reproduction. 

 Fio. A, Cidaris 9 X Lytechinusd'. Blastula, 18 hours. One cell in blastocoele; cells 



in wall seem to be in process of withdrawal. 

 Fig. B, Cidam 9 X Lytechinusd^. Gastrula, 23 hours. Seven cells in blastocoele. 

 Fig. c, Cidaris 9 X Lytechinus d' . Gastrula, 24 hours. 

 Fig. d, Cidaris 9 X Lytechinus d ■ Gastrula, 24 hours. 

 Fig. e, Cidaris 9 X Lytechinus d . Gastrula, 40 hours. 

 Fig. f, Cidaris X Cidaris. Blastula, 18 hours. 

 Fig. g, Cidaris X Cidaris. Gastmla, 18 hours. 

 Fig. h, Cidaris X Cidaris. Gastrula, 23 hours; 2 mesenchyme cells are in process of 



withdrawal from wall of archenteron. 

 Fig. i, Cidaris X Cidaris. Gastrula, 24 hours; 5 mesenchyme cells in blastocoele at 



plane of section. 

 Fig. j, Cidaris gastrula, 23 hours; X 180. Camera sketch of living gastrula; 3 mesen- 

 chyme cells in the blastocoele. 



