442 GENERAL HISTORY OF THE INFUSORIA. 



(XXXVII. 220), which contracts the substance of the ovary, and throws the 

 membrane into sharp folds. This membrane may likewise be detected with- 

 out the assistance of chemical reagents, where it is contracted below into an 

 outlet or duct opening in the cloaca. It forms a pellucid membranous bag, 

 which may be ruptm-ed by pressure, giving exit to its viscid contents ; and 

 Leydig asserts that the wall of the ovary is contractile, as the addition of 

 alcohol demonstrates. 



The substance of the ovary is called the ' stroma ' or protoplasm ; it has a 

 finely-granular appearance and a viscid consistence. It is usually of a milky 

 or a light-grey colour, and has interspersed in it, besides granules, numerous 

 clear bodies of a vesicular appearance (XXXYII. 1 e ; XXXVII. 7), but 

 which, Leydig says, are reaUy homogeneous. Williamson coimted between 

 20 and 30 in the ovary of Melicerta, varying in diameter from ^^^,j^) th to 

 y-gifj-jjth of an inch (XXXVII. 22). These, by development, constitute the ova 

 or eggs, and may be termed rudimentary ova. Within each a finely-molecular, 

 more or less opaque, and rounded body is perceptible (the nucleus), siuTounded 

 by a clear, transparent ring, apparently filled with fluid (the germinal vesicle) 

 (XXVII. 6, 7). " These are," writes Huxley, '' the geiTtiinal vesicles and 

 spots of the fature ova. Acetic acid, in contracting the pale substance, groups 

 it round these vesicles, vtdthout, however, breaking it up into separate masses. 

 It renders the nuclei more evident." This author further remarks, '*the 

 pale clear space is sometimes seen to be limited by a distinct membrane." The 

 measurements of the nuclei in Melicerta are, according to Williamson, from 

 g -^^ ^ th to j-jij-jjth of an inch in diameter. Within each nucleus are usually 

 from one to thi'ee clear spots — the nucleoli. The nucleolus, as understood by 

 Williamson, corresponds mth the nucleus in the preceding description, whilst 

 this last term is apphed by Huxley to the entii'e germinal body or rudi- 

 mentary ovum. 



Formation oe Ova, their Extrusion, and Development. — After fructifi- 

 cation, and preparatory to their transition into ova, the germinal spaces undergo 

 various changes in constitution and appearance. The germinal vesicle en- 

 larges, its nucleus disappears, and the ovum is indicated only by an ill- defined 

 transparent spot, which may, by pressure, be isolated as ^* a small spherical 

 cell about y^^y^th of an inch in diameter, having very thin pellucid walls, 

 and scarcely any visible cell- contents " (Williamson). Consentaneously with 

 these movements in the germinal space, the construction of the ovum pro- 

 ceeds by the attraction and separation of a portion of the surrounding proto- 

 plasm forming a yelk. The portion so appropriated is particularly rich in 

 granules which have previously congregated in the ovary, and now attracted, 

 it may be supposed, by the active vital action set up in the rudimentary 

 ovum. This abundance of granules produces a deeper colour and an increased 

 opacity in this portion of the ovary ; so that when, as Prof. Williamson re- 

 marks in the instance of Melicerta, this process of development proceeds in 

 the centre of the ovary, the latter organ appears divided by the incipient ovum 

 into an upper and a lower half. 



The consequence of these several changes is that the resultant ovum is of 

 considerable size (XXXVII. 170) and stands prominently outward from the 

 general surface of the ovaiy, acquiring at the same time an independent 

 character by the production of a limiting membrane about the viteUus or 

 yelk, called the vitelline or vitellary membrane. Huxley, indeed, does not 

 regard this as a distinct and specially produced covering, but as derived from 

 a portion of the enclosing membrane of the ovary, pinched off from the rest. 



Prof. Williamson enters into a comparison of the development of the ova 

 of Melicerta with that of the higher Mammalia, to show the close relationship 



