THE OVUM IN THE AMPHIBIA. 219 



the envelope is greatest during the first three hours, and rightly regarded the occur- 

 rence as connected with impregnation, and with this opinion made experiments to 

 test their views. They placed ova taken from the oviducts in ink, and found the en- 

 velopes blackened with the imbibed fluid* ; but they remark " bientot cette imbi- 

 bition s'est arretee a cause de la reaction chimique de 1'encre qui coagulait la matiere 

 nmqueuse." Afterwards they employed the blood of the Frog mixed with water, and 

 found the envelope deeply reddened when immersed in it ; and thence concluded 

 that the envelope in its normal condition admits of the entrance into it of solid par- 

 ticles of matter held in suspension in the fluid. But each of these experiments 

 appears to be open to a different explanation. It is probable that the chemical action 

 of the ink, by altering the condition of the envelope, allowed of the admission into it 

 of solid particles only in proportion to the change in the tissue ; and that the colour 

 given to the envelope by frog's blood was due to particles of colouring matter which 

 adhered to the surface, rather than to the admission of these into the substance of the 

 tissue. This conclusion is founded on the following trials. 



The imbibition of water by the covering of the egg being so distinctly marked, I 

 had intended, like the authors mentioned, to endeavour to ascertain whether coloured 

 water could be as readily absorbed as pure water. Through accident, however, I 

 omitted to put this question to the test until late in the season of last year, and after 

 the whole of my frogs had spawned. But having placed several ova in rectified spirit 

 for future examination, at the moment of passing them from the body of a frog, I 

 determined to test the result of the immersion of these in coloured fluid, although 

 well aware of the correct objection that would be made, that the experiment must of 

 necessity be inconclusive. I put some of these into a solution of carmine in water, 

 and watched the result. The envelope, which, while the egg was in spirit, was white, 

 opaque, and adhered closely around the yelk membrane, began to imbibe and expand 

 the instant it was placed in the solution ; and at the expiration of an hour there 

 seemed reason to believe that the trial had succeeded. The envelope was much en- 

 larged, and the fluid had penetrated into it, carrying with it some colouring matter ; 

 as on carefully washing the surface repeatedly in clean water, to remove the deposit 

 on the exterior, the substance of the interior was seen to be coloured, and it was evi- 

 dent that the colouring matter had penetrated as fay as the deepest or thick layer of 

 the envelope. This result appeared to favour the view that the spermatozoon enters 

 the ovum. When the ova were again examined at the end of the third day's immer- 

 sion, they were of a deep red; the deepest stratum of colour being then between the 

 vitelline membrane and the thick or innermost layer of the envelope. The entire egg 

 presented the appearance of a globule or bead of red glass, with a dark red centre, 

 surrounded by a lighter-coloured halo. The eggs were then thoroughly washed and 

 placed in clean water. At the end of six hours part of the colouring matter had 

 again been removed from the interior, and the eggs were of a less deep hue, and the 



* Loc. tit., ii. p. 132. 



2 F2 



