4<>2 



GENERATION. 



constitutes the cicatricula or germinal disc is 

 easily seen in the larger ovula, occupying 

 almost always the same position on the surface 

 of the yolk, somewhere near the pedicle of 

 the ovarian capsule. When the cicatricula is 

 examined carefully in the ovulum, a small 

 dark round spot is perceived in its centre, the 

 relations of which to the first production of the 

 foetus are very important. This little dark 

 spot was discovered by Purkinje to contain im- 

 planted in the centre of the cicatricula a minute 

 transparent vesicle filled with fluid. He farther 

 shewed that during the passage of the ovulum 

 from the capsule of the ovary into the infun- 

 dibulum of the oviduct, this little vesicle dis- 

 appears, being probably burst, and leaves in 

 its place a thin and tender transparent mem- 

 brane. The vesicle of Purkinje, as it is called 

 from its discoverer, occupies then the centre of 

 the germinal spot, and it is in the transparent 

 membrane left in its place when the vesicle is 

 dispelled that the first rudiments of the foetus 

 afterwards make their appearance. Hence the 

 vesicle has also received the name of germinal 

 vesicle, a most appropriate term, since it may 

 be regarded as the more immediate seat of the 

 germ or germinating faculty of the egg. 



The Purkinjean or germinal vesicle exists in 

 the smallest as well as in the more advanced 

 ovula of the fowl's ovary, and it is proportion- 

 ally much larger in small than in large ovula. 

 In the very small ovula it is not, as in the riper 

 ones, situated on the surface of the yolk, but 

 towards the centre of that body; and as the ovu- 

 lum advances to perfection, the germinal vesicle 

 gradually approaches more near the surface, 

 and becomes more prominent on the surface of 

 the cicatricula. In ovula less than two lines 

 in diameter the vesicle is usually unconnected 

 with the germinal layer or cicatricula, but in 

 those of four lines in diameter it is already 

 placed in the middle of the germinal spot. 



In all oviparous animals a vesicle, simi- 

 lar to that now described in the common fowl, 

 occupies the central part of the germinal layer 

 so long as the ovulum remains in the ovary, 

 and undergoes the same rupture and other 

 changes at the time of the discharge of the 

 ovulum from the ovary.* 



In turning now to mammiferous or vivipa- 

 rous animals, it may be remarked in the first 

 place, that although the extremely minute size 

 of the body discovered by Baer to be constantly 

 present in the ovarian vesicle prevents us from 

 observing it with ease, and establishing with 

 certainty its analogy to the yolk and its accom- 

 panying parts in the egg of the fowl before deve- 

 lopment begins ; yet after the commencement 



* Purkinje's description of this vesicle was first 

 given in his excellent " Symbolae ad ovi ovium 

 historiam ante incubationem, Vratisl. 1825," and 

 second edition at Leipzig, in 1830. Baer contri- 

 buted in his " Epistola" many important facts 

 concerning its existence and changes in other ovi- 

 parous animals. Coste, Valentin, and Wagner 

 have since added several observations. We may 

 state here that the bursting of the vesicle does not 

 occur in all oviparous animals exactly at the time 

 of the escape of the ovulum from the ovary, but 

 nearly about the same time. 



of foetal formation, the early changes which this 

 body undergoes prove its correspondence with 

 the ovum of birds in a most satisfactory manner. 

 We have already, however, stated the reasons 

 for regarding the vesicle of Baer as the ovulum 

 of mammalia, and need not now recapitulate 

 them. We shall only remark that although 

 the vesicle of Baer and ovulum of birds differ 

 widely in size, that vesicle appears to contain 

 the same essential parts of the egg belonging 

 to birds and other oviparous animals, viz. a 

 fluid granular mass or yolk enclosed by an in- 

 vesting membrane, and furnished also with a 

 more compact granular layer situated on the 

 surface of the yolk, but also enveloped by its 

 membrane, in which the rudiments of the 

 foetus first appear, and which is, therefore, the 

 germinal layer of the mammiferous ovum. 



The membranes of the ovarian vesicle in 

 mammalia and the capsules of the ovary, in 

 the fowl are corresponding parts, and the prin- 

 cipal difference between the ovarian cavities 

 containing ovula in oviparous animals, and 

 those of viviparous animals, consists in this, 

 that in the latter the ovulum (the vesicle of 

 Baer) is placed in the granular proligerous disc, 

 and has all the fluid of the vesicle interposed 

 between it and the coats of this cavity. 



At the time when Baer first discovered the 

 ovulum of mammalia, there was still wanting, 

 in order to complete the proof of its analogy with 

 the ovulum of birds, the observation of the ger- 

 minal vesicle (vesicle of Purkinje) within it. 

 This additional proof has been supplied within 

 the last few years by the researches of T. W. 

 Jones, Coste, Purkinje, Valentin, and Wag- 

 ner, which we have ourselves confirmed. 



The germinal vesicle of the very small ovu- 

 lum of quadrupeds is of course a most minute 

 object, and in fact it can only be seen with a 

 good microscope ; but in favourable circum- 

 stances it is nevertheless quite distinct, and 

 the investigations above referred to, conjoined 

 with analogical evidence, make it highly pro- 

 bable that the little vesicle found within the 

 ovulum of viviparous animals occupies the 

 place in which the foetus first makes its ap- 

 pearance, and that at the time of the passage 

 of the ovulum from the ovary to the Fallopian 

 tube this little vesicle is burst, and undergoes 

 analogous changes to those which have been 

 noticed in the fowl.* 



In birds the shell with its lining membrane 

 forms the external covering of the egg ; and in 

 all oviparous animals a similar external enve- 

 lope (besides the membrane enclosing the yolk) 

 is to be found, though varying greatly in thick- 

 ness, consistence, and structure in different 

 animals. The ovum of mammalia at the time 



* In his " Epistola/' published in 1827, Baer 

 compared the vesicle he had discovered within the 

 Graafian vesicles of the ovary to the vesicle which 

 Purkinje had in 1825 discovered in the cicatricula 

 of the fowl's yolk : erroneously as we think ; for 

 the facts mentioned above are sufficient to disprove 

 any such analogy. For the sake of clearness we 

 here subjoin a tabular view of the parts which 

 correspond with one another in the bird and quuil- 

 ruped. 



