EXPLANATION OF PLATE X— continued. 



concentric layers round the nucleus of the germ producer («'). The germs 

 of the latest generations are still lodged in the protoplasm of the germ- 

 producing cell ; their nuclei are still homogeneous, and their contour is 

 only feebly defined. 



jTiG. 6. — The division of the two cells of a vermiform germ into four 

 similar cells Dicyemella Wageneri (imm. 10, oc. 4). 



Fig. 7. — The division of the previous cells into seven, of which one 

 is large whilst the rest are small. Dicyemella Wageneri (imm. 10, 

 oc. 4). 



Fig. 8. — Gastrula of vermiform embryo of Bicyemina K'ollikeriatia, 

 prep, in acetic acid (obj. immers. 10, oc. 2). 



Fig. 9. — Vermiform larva of Bicyemina Kollikeriana (imm. 10, oc. 2). 



Figs. 10 — 15 apply to the development of infusoriform embryos. 



Fig. 10. — Spherical germ of Dicyemella. The nucleus is striated, and 

 exceedingly clear strite follow the meridional lines and converge towards 

 the two small refractive corpuscles situated at the poles. 



Fig. 11. — An elliptical germ, with an elliptical nucleus ; at the two 

 extremities of the great axis the same polar corpuscles are seen. Near 

 these is a layer of dark substance which pei'haps foreshadows the division 

 of the two halves of an equatorial plate. These are the nuclear discs ; 

 and between them are the meridional strise. The median plane of the 

 ellipse is marked by a series of dark points. 



Fig. 12. — A germ in the same stage of development, seen along its 

 main axis. In the centre is the polar corpuscle surrounded by the polar 

 disc, and forming with it the derived pronucleus. This figure shows that 

 the striation is due to filaments which are only found on the surface of the 

 nucleus. 



Fig. 13. — A more advanced stage of development. The derived pro- 

 nucleus is surrounded by a layer of transparent substance, easily seen at 

 the side and at the periphery (the engendered pronucleus). 



Fig. 14. — A germ still further advanced. The intercellular plate ex- 

 tends to the surface. The derived pronucleus is attached to the engen- 

 dered pronucleus. 



Fig. 15. — Germ divided into four, the fusiform nuclei on the point of 

 dividing. 



Fig. 16. — Vermiform embryo in which the refractive bodies have just 

 appeared. There are two large superficial cells {p) lying next each other, 

 which form eventually the parietal cells of the urn, and two median cells 

 («), the superficial cells of the urn ; the two cells (r) immediately in front 

 of the preceding cells have given rise to the refractive bodies. 



Fig. 17. — An embryo rather more advanced, a. In optical section ; b, 

 inferior face formed by the future ciliated cells ; p, the parietal cells ; c, 

 superior face ; r, cells of the refracting body ; *, superficial cells of the 

 urn ; i, granular body. 



Fig. 18. — Infusoriform embryo of Octopus vulgaris. The cover is not 

 perfectly formed. The granular bodies are still in contact with the sur- 

 rounding medium. The cap is rudimentary. The granular bodies are 

 still uninuclear cells. ( p) The two parietal cells forming the capsule 

 ultimately ; {s) the superficial cells modified to form the cover ; {i) poly- 

 nucleated cells of the interior of the urn. 



Fig. 19. — Infusoriform embryo of Dicyema typtis dissociated by mace- 

 ration in haematoxylin for three hours. The refractive bodies have 

 burst ; ectocysts ice) ; endocysts (ci) ; at the sides granular masses which 

 have escaped from the internal capsules ; ( p) the border of the urn ; {i) 

 granular bodies (polynucleated cells) enclosed in the urn ; {c) isolated 

 cells of the ciliated body. 



