DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY 305 



PART I. 



On the origin and structure of the true Ovisac, a vesicle common to all vertehrated 

 animals, but hitherto described as the innei' membrane of the " Folliculus Graafi- 

 anus" in Mammalia, and by some authors denominated the " chorion^ in other 

 yertebrata ; on the real nature of the " Folliculus Gr^aqfianus^'' and its relation to 

 the calyx of the Bird; on the Germinal Vesicle and its contents, as the most primi- 

 tive portion of the ovum ; on the order of formation of the several other parts of 

 the Ovarian Ovum; and on the true Chorion of Mammalia as a structure superadded 

 within the ovary. 



1. The early structure of the ovisac in the Mammalia may be seen in either quite 

 young animals, or in those that have lately reached the state of puberty, when this 

 vesicle and its contents are in the full vigour of formation. To find it in a young 

 animal, I am in the practice of placing under the microscope thin slices cut from the 

 surface of the ovary ; or in animals after puberty, the outer portion of a large Graaf- 

 ian vesicle. Compression is of course required, but it should be very gradually ap- 

 plied. Great pressure is rarely wanted, and for the most part very undesirable. 



The Ovisac of the Mammalia. 



2. This vesicle originates in the proper substance (stroma) of the ovary-}-. Its 

 general appearance when first formed is as a minute, pellucid, often yellowish sac, 

 having an elliptic form, and plaits or folds in its contour (Plate V. figs. 4. to 12. h.). 

 The ovisacs are found in groups, and those belonging to the same group are often of 

 nearly the same size (figs. 4. 10. h.), though this is by no means constantly the case. 

 Sometimes a group occupies more than the whole field of view ; some of the ovisacs, 

 more deeply imbedded than the rest, htiving the appearance of obscure pellucid spaces, 

 others being superficial and distinct (fig. 4. h.). Could a fragment of amygdaloidal rock 

 be made translucent and its amygdaloidal portions luminous, it would present an 

 appearance not unlike that of a collection of minute ovisacs in the parietes of a Graaf- 

 ian vesicle from one of the Ruminantia. In Carnivora, such as the Dog and Cat, I 

 have met with them having less of interstitial substance, innumerably heaped together 

 (Plate V. figs. 6. 10., Plate VHI. tig. 68. h.). The surface of an ovary presents to the 

 naked eye perhaps ten, twenty, or fifty Graafian vesicles, while the microscope dis- 

 closes myriads ; so that some parts bear no mean resemblance to the roe of fishes. 



3. The ovisac at its origin, as just said, has an elliptic form. As its size advances 

 it becomes more spherical, and is often met with somewhat tapered at one end 

 (Plate VIII. fig. 74. h.). When it appears round in an early stage, this is perhaps 

 owing to its being transversely situated under the compressor. 



t It appears sometimes to originate in the parietes of a Graafian vesicle. Ovisacs met with in this situa- 

 tion will be more particularly referred to hereafter as parasitic ovisacs. 

 MDCCCXXXVIII. 2 R 



