310 DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 



that so great a difference in magnitude as this in the same animal should be per- 

 manent ; and if this be admitted, it follows that after formation the germinal vesicle 

 increases in its size. It may therefore when first formed be much minuter than the 

 smallest of those which I have figured. 



20. I find that the germinal vesicle of a mature ovum of the Rabbit does not ge- 

 nerally exceed the 50th of a Paris line-j~ in diameter ; and this being just the size of 

 the largest of those from the same animal in fig. 1, it seems probable that in the 

 Rabbit J the vesicle does not increase much in magnitude after this early period. 



21. It is proper to state, that in certain animals I have met with ovisacs much mi- 

 nuter than some of the objects (still uncovered by an ovisac) in fig. I. This admits 

 of explanation by supposing either that the germinal vesicle is much smaller, that its 

 granulous envelope is much thinner, or that the period of formation of the ovisac, in 

 reference to the size of the germinal vesicle, is much earlier in some animals than in 

 others. There may also be a difference in different animals in each of these respects. 



22. From the observations of Professors Baer and R. Wagner in invertebrated 

 animals, and those now recorded from researches in two classes of the Vertebrata, I 

 think I am warranted in concluding that the germinal vesicle and its contents con- 

 stitute throughout the animal kingdom the most primitive portion of the ovum. In 

 Birds and Mammalia the succeeding process appears to consist in the accumulation 

 around the germinal vesicle of oil-like globules and peculiar granules. See the first 

 column in the Table, page 304. 



The manner of Origin of the Ovisac. 



23. Around the elliptic envelope of granules and oil-like globules just described as 

 proper to the germinal vesicle of Mammalia and Birds in an early stage, I find that 

 there is formed a membrane, seen in Plate V. fig. 20. h. If the contents of this mem 

 brane, which is still incomplete, be compared with the granules (g) in fig. 19, these ob- 

 jects will be found to be the same ; in farther proof of which it may be added that 

 they were met with, as well as those in fig. 18, all lying together^. The membrane, 

 h, is shown completely formed in nearly all the other figures of the same Plate. This 

 is the membrane which I have already described in Mammalia, and intend describing 

 in other Vertebrata, as the ovisac. See the second column in the Table, page 304. 



The order of Formation of the several parts of the Ovarian Ovum. 



24. After the formation of the ovisac, the germinal vesicle, as already said (12.), is 

 generally for a short time concealed. This is perhaps partly referrible to minute oil- 

 like globules being mixed with the peculiar granules of the ovisac, and causing great 

 refraction. Liquefaction, however, of some of the granules appears to take place, or 

 a fluid from some other source is added, and then the germinal vesicle is seen in or 



t = -s-^-j- of an English inch. J And probably in Mammalia generally. 



§ All of these objects, moreover, were lying among newly-formed ovisacs. 



