JOURNAL OF MICROSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XIII, 



Illustrating Edouard van Beneden's Contributions to the 

 History of the Germinal Vesicle., and of the First 

 Embryonic Nucleus. 



Figs. 1 — 19. — AsteracantMon ruhens. 



Fi&. 1. — An ovarian e^g arrived at maturity, but still showing its germinal 

 vesicle at the centre of the yolk. 



Figs. 2 — 8. — Represent one and the same egg in which the different phases 

 of the disappearance of the germinal vesicle have been observed. Subse- 

 quently I saw iu the same egg the retraction of the yolk, the yolk assume 

 an almost spherical form, the polar globules appear in the peri-vitelline 

 liquid, the first embryonic nucleus appear in the centre of the yolk, and 

 finally the yolk divide into two spheroids. I have not figured these last 

 modifications, both because I have not described them in the text and because 

 I have never been able to observe either how the directive bodies are formed 

 or how the first embryonic nucleus is produced. I quote these later modifica- 

 tions as having been produced under my eyes in the same egg in which I 

 traced the series of changes here figured (2 — 8), for this reason — that these 

 facts prove that the egg in question was normal and developed in a normal 

 manner, although its oval shape is exceptional for a ripe egg. All the other 

 eggs iu which I saw the germinal vesicle disappear were spherical or approxi- 

 mately so. 



Fig. 2. — By the side of a nucleolus which has an uneven outline, and in 

 which there is only a single vacuole, are still seen pseudo-nucleoli. 



Fig. 3.T-The pseudo-nucleoli have disappeared and the nucleolus appears 

 composed of a certain number of globules collected round a clear transparent 

 central corpuscle. 



Fig. 4. — The nucleolus is broken up into a large number of fragments, the 

 refractive power of which gi-adually diminishes, at the same time that the 

 outline of the vesicle becomes less distinct. 



Fig. 5. — The last traces of the nucleolus have disappeared. The membrane 

 of the germinal vesicle has burst, and the contents escape by the opening. 



Figs. 6, 7. — The contents of the vesicle are in great part escaped, the mem- 

 brane of the vesicle shrinks and the volume occupied by the germinal matter 

 diminishes. 



Fig. 8. — The last traces of the germinal vesicle have disappeared. 



Fig. 9. — Germinal vesicle of an egg similar to that which I have repre- 

 sented in fig. 1, after its escape from the yolk. In the nucleoplastic network 

 arc seen the nucleolus and the pseudo-nucleoli. Observed with Hartnack's 

 No. 9 immersion objective. 



