306 DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 



4. Its size when first formed is exceedingly minute. I have found it in several 

 orders of Mammalia measuring not more than the 50th of a Paris line-f- in length. 

 Such is the case, for instance, in the Ox (Plate V. fig. 4. h.) ; so that a cubic inch would 

 contain upwards of two hundred millions, not merely of the elements of the ovum, 

 but of the ovisacs of this animal. The minuteness of these vesicles indeed is almost 

 incredible. In the Dog I have seen them measuring only the 100th of a Paris line:j: 

 (Plate V. fig. 9. ^.), thus little more than one third the long diameter of a blo6d-granule 

 (red particle) of the Proteus anguinus^. 



5. The ovisac is more or less pellucid according to its size; being most so in the 

 early stages of formation, and becoming merely translucent as development advances. 

 This is partly owing to the gradual addition of an external covering or tunic, to be 

 hereafter more particularly mentioned (24. 25.). It continues however in all its 

 stages more translucent than the substance in which it lies. 



Cavity in ivhich the Ovisac is often found. 



6. More particularly considered, the situation of the ovisac in its early state is often 

 found to be a cavity (Plate VIII. fig. 69.), sometimes, as already said, in the proper 

 substance (stroma) of the ovary, and sometimes in the parietes of a Graafian vesicle. 

 It lies loose in, and unconnected with the walls of, its containing cavity. That it 

 does so is shown in the figure just referred to (fig. 69.), which presents an ovum, f 

 escaped from the ovisac, h, and lying external to the latter in the cavity. (This 

 change in the situation of the ovum was seen to take place on the ovisac being burst 

 under the microscope by means of the compressor ; when the ovum became squeezed 

 into an elliptic form. This has repeatedly occurred ||.) The cavity in which the ovisac 

 lies, may sometimes be found after the latter has been pressed out of it. 



The proper membrane of the Ovisac in Mammalia. 



7- In ovisacs of the minutest size this membrane is perfectly transparent ; yet in 

 and near to its contour it looks as though it were " concentrically lamellar"^ (Plate V. 

 figs. 9. 11. 12. h.). I apprehend this appearance to arise from plaits or folds occur- 

 ring in the membrane under pressure even at that early period, and indicating great 

 susceptibility of distention. Its thickness is relatively very considerable in the smaller 



t That is, about the 562nd of an English inch. See the table of measurements (118.), for a simple mode of 

 reducing the fraction of a French line ('") into the fraction of an English inch. 



X = YY2 J °^ ^"^ English inch. 



§ Through the kindness of Professor R. Wagner I possess a living specimen of this animal, and have re- 

 peatedly confirmed the observation first made by him, that the red particles of its blood measure in some in- 

 stances ^Q of a Paris line in length, and that they are visible with the naked eye, being larger than those of 

 any other animal the blood of which has been examined. 



II The ovum in this instance was ruptured before escaping, and left its germinal vesicle, c, behind. 



^ The appearance liere referred to has been observed by R. Wagneb, who uses the above exi^ression in de- 

 scribing what he considers minute Graafian vesicles (Beitrage, &c., S. 28.). 



