PLATE IV. 

 STAGES OF FERTILIZATION, SEGMENTATION, AND BLASTULA. 



(Preparations magnified about 15 diameters. All drawings from fresh material. Figs. 22 to 28 from camera drawings 

 of embryos which had been removed from the egg and viewed as transparent objects.) 



Fig. 1 8. Late stage of fertilization. The oblong shape of the germinal area is due to artifact. The 



preparation illustrates the number and size of the entrance pits of spermatozoa and the 



extent of the marginal groove. 

 Fig. 19. Later stage of fertilization. This indicates the extent of the marginal groove and the 



difference in size of the entrance pits of the spermatozoa. 

 Fig. 20. Stage showing in surface view a single furrow. As already noted, however, this stage is 



not one of first segmentation, since it contains several segmentation nuclei. Surrounding 



the germinal area is a narrow groove margined outwardly by eminences containing sperm 



nuclei. 



Fig. 21. Stage similar to foregoing, but showing at the surface four '' blastomeres." 

 Fig. 22. Stage of early segmentation. Here the marginal areas containing sperm nuclei are far 



less conspicuous. 



Fig. 23. Stage similar to the preceding. 

 Fig. 24. Stage of segmentation. 



Fig. 25. Stage of late segmentation. Blastomeres in resting stage. 

 Fig. 26. Stage of late segmentation. 

 Fig. 27. Stage of late segmentation. The darker color of the central blastomeres indicates a 



greater depth in this region of the germ. 

 Fig. 28. Blastula. In this stage inter-blastomeral lines were traced over the light-colored circum- 



germinal ring. 

 Fig. 29. Blastula. Viewed as an opaque object, and showing a sharply marked boundary between 



the blastoderm and the circumgerminal ring. 



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