322 DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 



tunic, between which and the surrounding mass there is much less adhesion than 

 there is between the granules of either among themselves. Having never failed to 

 find this tunic enveloping mature ova in the«Mammalia, I believe its presence to be 

 constant and essential ; and I am also of opinion that in form, substance, and situa- 

 tion it is essentially the same throughout this c^ass of animals. J propose to call it 

 the granulous tunic of the ovum, tunica granulosa. 



65. At a certain period this tunic, in some animals at least, is seen to have tail-like 

 appendages consisting of granules precisely similar to its own (Plate VII. fig. 63. g^.). 

 These appendages appear to be frequently four in number, and their direction corre- 

 sponds to that of other granulous cords or bands to be presently described (86.). 

 They are not very distinctly seen while witjiin the Graafian vesicle, and in this situa- 

 tion I have not figured them. 



66. Should the existence of this structure as a tunic be confirmed by the observa- 

 tions of others, it will no longer be surprising that the outer line in the double contour 

 of the thick chorion remained so long unseen, and that this thick membrane was re- 

 garded as a " zone," " halo," or " pellucid space." For the same reason it will be ob- 

 vious why it has been found so difficult to free the ovum from the surrounding mass 

 of granules ; though I find that these granules do not adhere to the chorion they in- 

 vest, so closely as to each other. 



67. The tunica granulosa may be obtained in an uninjured state, either by care- 

 fully opening a Graafian vesicle of considerable size, and receiving its fluid and gra- 

 nulous contents into a watch-glass (Plate VII. fig. 61.^*.), or by bursting one of 

 middling size under the microscope with the compressor, when the escape of the 

 ovum in its tunica granulosa may be observed. In both cases, however, portions of 

 another granulous structure (the retinacula, to be presently described (80.),) gene- 

 rally escape adherent to it. This tunic is also very distinctly visible in situ, as in 

 Plate VI. figs. 39, 40. and Plate VII. figs. 55. to 59. g\ 



68. I have met with the ovum when immature, and. apparently just after the for- 

 mation of the chorion (Plate V. fig. \7.f.), at which period the tunica granulosa does 

 not exist ; and in subsequent stages its gradual formation may be observed, as in 

 Plate VI. fig. 41. g^., where its thickness is inconsiderable and its contour irregular. 



69. The tunica granulosa, as already said, is spherical. In whatever direction it is 

 viewed, whether as in Plate VII. fig. 61. g^., or in any other of the figures of this 

 Plate, there is perceptible no diff'erence in its form ; the distinctness with which the 

 contained ovum is seen, depending, not on the absence of the investing granules at 

 any part of its periphery, but on the extreme transparence of these granules. The 

 flattening at one side which this tunic may undergo, and the entire deprivation of 

 granules, perhaps, at one point of the surface of the chorion, by pressure against the 

 membrane of the ovisac, I shall have to notice (88.) -f*. 



t I shall presently describe a structure supposed, as already said (56.), by Professor Baer to be "discoid" 

 in its form. In the I>o^, however, this author has obviously figured as the " discus proligerus" the structure 



