296 DR. J. B. GATENBY ON THE GERM-CELLS AND 



Fig. 7. Ordinal amoeboid cell of the mesoglea of Grantia ; it lies in the mesoglea! fibrous 

 tissue (MES). At M are its mitochondria. There is a distinct ectoplasm. 

 Ch.K., I.H. 



Fig. 8. An oogonium of the mesoglea ; the arrows point to the nearest gastral cavities. 

 Ch.K. 



Fig. 9. Full-grown oocyte of Grantia with nearest accompanying choanocytes, in one of 

 which (at S) a sperm has entered. This oocyte shows the typical endoplasmic 

 region with the granular inclusions and the ectoplasmic part; on the left is a 

 pseudopodium. At M the mitochondria, and at Y the yolk-granules, are evenly 

 distributed; no animal or vegetative pole could be identified by reason of unequal 

 distribution of the cell-granules. Ch.K., I.H., & K. 



Plate 20. 



Fig. 10. Oocyte at lower magnification to preceding, showing large sperm just after entry. 

 The cell-granules tend to approach the region of entry of the sperm. 



Figs. 11 & 14. Gastral side of oocyte showing sperm-carrying cell lying upon the former; 

 in both sperm-carrying cells the nucleus is flattened on one side owing to the 

 pressure of the contained sperm ; in fig. 11 the sperm will enter middle-piece 

 forwards, in fig. 14 middle-piece backwards. The ectoplasm in the region of 

 entry of the sperm is interrupted. In fig. 1 1 yolk is uot shown. Ch.K. 



Fig. 12. Sperm just entering, acrosome first. Shows protoplasmic continuity between 

 sperm-carrying cell and oocyte. Ch.K. 



Fig. 13. Later stage, polar body at PB, and female pronucleus growing (FPN). Sperm- 

 path marked by pale chromophobe area (VAC). Note large size of mitochondria 

 (M). Macromitosome of sperm disappeared, but may be represented by granules 

 at x. Ch.K., I.H., & K. 



Fig. 14. See description to fig, 11. Ch.K., I.H., & K. 



Fig. 15. Flattened sperm-bearing coll, which was compressed by egg, and so looks abnor- 

 mally large. Its nucleus is of the open type. Ch.K. 



Fig. lb'. Sperm just after entry ; its cell-wall has disappeared, macromitosome still complete, 

 and sperm-cytoplasm furred at edges but still distinguishable from egg-cyto- 

 plasm by its greater chromophility and stringiness. Acrosome pale. Nucleus 

 nearly spherical and has grown. Sperm-carrying cell at SCO, with open 

 nucleus. 



Fig. 16. Stage in fertilization after that in fig. 12. The sperm cell-wall has disappeared and 

 its stringy cytoplasm is still distinct from that of the egg. The acrosome (AC) 

 is becoming faint, Ch.K. 



Fig. 17. Later stage just after breaking up of sperm middle-piece or macromitosome (MAM). 

 This preparation was fixed by Champy-Kull and stained by Heidenhain's Iron 

 Alum Hematoxylin. Note that nuclei of sperm-carrying cell (SCC) and those 

 of choanocytes (NCH) stain alike. Ch.K., I.H. 



Plate 21. 



Fig. 18. The two pronuclei side by side prior to last stage of fertilization. Note even dis- 

 posal of inclusions (M & Y). The sperm-carrying cell (SCC) is degenerating in 

 this case. Yolk added from another preparation at a similar stage. Ch.K. & K. 



Fig. 19. Two-cell stage, showing even disposal of mitochondria. Upper blastomere not cut 

 across middle. Ectoplasm evenly divided. Ch.K., I.H. 



Fig. 20. Four-cell stage, showing continued subequal distribution of the mitochondria and 

 of ectoplasm. H. W.A., I.H. 



