OVUM. 



[93] 



It is towards the centre of the dark super- 

 ficial part of the yolk that the first changes 

 of embryonic development always take place ; 

 and it is apparent that this dark part corre- 

 sponds more immediately to the germinal part 

 of the yolk. It is beneath the central part of 

 this dark covering that the germinal vesicle 

 is situated in the ripe ovum. When taken 

 from the ovary previous to impregnation, the 

 ova float in water indifferently as regards the 

 position of their parts ; but after impregna- 

 tion, when the imbibition of water allows of 

 the free rotation of the yolk within its cover- 

 ings, it is invariably found that the dark or 

 germinal part of the yolk is directed upwards, 

 and the whiter or grey spot downwards ; 

 a circumstance by which the difference 

 between the fecundated and the unfecundated 

 ova may readily be detected. We mny dis- 

 tinguish therefore, as the upper, dorsal, or 

 germinal pole of the ovum, the central point 

 of the dark part, and name the opposite point 

 in the centre of the light-coloured space the 

 lower or ventral pole. 



The thickness of the dark layer of substance 

 which covers the upper part of the ovum is 

 throughout its greater part considerable ; viz. 

 about one-eighth to one-tenth of the whole 

 diameter of the yolk. It thins off somewhat 

 at its edges below. Within this darkest layer 

 the colour of the yolk-substance is slightly 

 shaded off into the grey substance of the in- 

 terior : the consistence of the inner substance 

 is less than that of the superficial layer, and 

 it contains a cavity situated considerably 

 nearer the upper than the lower surlace of 

 the yolk, in which the germinal vesicle is 

 situated. This vesicle is not perfectly sphe- 

 rical in its form, but somewhat flattened from 

 above downwards, and it is surrounded by a 

 peculiar mass of fine granules. 



The yolk-substance contains no cells nor 

 large corpuscles ; the greater part of it consists 

 in the mature state of peculiar flat or tabular cor- 

 puscles, the largest of which are about 2 -^oo" 

 in diameter. Most of these are quadrangular 

 in shape, but somewhat rounded on the edges 

 and at their angles. There are also numerous 

 smaller particles of the same kind of every di- 

 mension from that already stated down to the 

 smallest granules, and with some variation of 

 shape, together with a considerable amount 

 of molecules of very minute size, of which 

 those in the darker part have the appearance 

 of pigment granules. These last are accumu- 

 lated in greatest quantity towards the surface ; 

 but they do not corstitute a separate layer, 

 being rather interspersed with the tabular cor- 

 puscles. There are also to be seen in the 

 upper or germinal part a few rounded corpus- 

 cles, somewhat larger than the tabular particles, 

 which seem to be formed by the aggregation 

 of smaller molecules ; but these have no ex- 

 ternal envelope nor clear nucleus, and only 

 bear a distant resemblance to the cells which, 

 after impregnation, are formed in the germinal 

 part of the yolk-substance. 



The peculiar quadrilateral tabular corpuscles 

 refract light strongly, so as to present distinct 



outlines ; they have also considerable firmness, 

 resisting pressure, but by force may be broken 

 up somewhat in the same manner as would 

 occur in small plates of wax. From this cir- 

 cumstance they have generally been regarded 

 as of a fatty nature, and were described by 

 Vogt as stearine tables; but Vircbow*, from 

 a careful investigation of their reaction with 

 different substances, throws a doubt upon this 

 view, and is more inclined to regard these cor- 

 puscles, both in Batrachia and in the ovum of 

 the carp-fish, as composed of some albuminous 

 or protein principle, the exact nature of which 

 he has been unable to determine. He admits 

 that they may also contain some oil. They 

 are probably very antilogous to the larger 

 firm angular particles which were first de- 

 scribed "by J. Miiller as forming the greater 

 part of the yolk-substance in the Sharks and 

 Rays, and which also exist in the ova of 

 Cephalopodous Mollusca.-f- 



The germinal vesicle of the Batrachian ovum 

 is of very large proportionate size. According 

 to Vogt, in the Alytcs obstetricans its diameter 

 is nearly equal to one-third of that of the entire 

 yolk mass. In the common frog and toad it 

 is somewhat less, but nearly of an inch. 

 This vesicle may be obtained separate for 

 examination by breaking open the yolk care- 

 fully under water ; but it is much easier to 

 observe its position, form, and structure in 

 the ovum which has been hardened by some 

 re-agent, a plan which has been successfully 

 adopted by a variety of observers. Cramer J 

 recommends for this purpose alcohol, or more 

 particularly dilute chromic acid; Newport $ 

 employed alcohol, as I also have done with 

 success ; more recently Remak || states that 

 he has found a mixture of a solution of sul- 

 phate of copper with alcohol, to which a few 

 drops of rectified wood spirit are added, pecu- 

 liarly fitted to give the proper consistence to 

 the various parts, without inducing any de- 

 structive change in their structure or appear - 

 ance. All observers agree that there is 

 scarcely any other animal in which the re- 

 lations of the germinal vesicle to the other 

 parts of the yolk can be more favourably in- 

 vestigated. 



The enclosing wall of the vesicle is of ex- 

 treme tenuity, so thin, indeed, that some have 

 doubted its existence. I have been able, how- 

 ever, to distinguish the double outline of its 

 thickness with a good magnifying power of 350 

 diameters. The outer surlace of the vesicle 

 is not always of a regular circular or spherical 

 form, but often presents within the yolk, both 

 at earlier and more advanced stages, a notched 



* Zeitsch. fiirWissensch. Zool. vol. v. p. 241. 



f See J. Miiller, iiber die Glatten Hai des Aristo- 

 teles, 1842, p. 36. 



J Bemerk. iiber das Zellenleben in der Entwick. 

 des Froscheies, in Miiller's Archiv. 1848, p. 20. 



Researches on the Impregnation of the Am 

 phibia, First Series, in Phil. Trans, for 1851, 

 p. 1G9. et seq. 



|| Untersuch. iiber die Eutwickel. der Wirbel- 

 thiere, Berlin, 1855, p. 127. 



