OVUM. 



Fig. 52. 



Microscopic structure of the elements of the yolk 

 and ovarian ovum of the fowl. 



A. Large granular corpuscles of the yellow part 

 of the vitellus ; one of them quite spherical, as they 

 are seen when free ; two others angular from mutual 

 compression, from a boiled yolk. 



B. Various corpuscles found on the confines of the 

 3'ellow yolk and the cavity and canal, showing 

 transition forms to the nest set. 



c. Clear vesicles containing oil globules and de- 

 tached oil globules of various sizes from the cavity 

 and canal. 



r>. From the cicatricula ; a, various-sized granules 

 and globules forming the vitelline disc of the yolk 

 before its discharge from the ovary ; b, the organised 

 nucleated cells forming the upper layer of the 

 cicatricula in a laid egg ; c, larger cells of the lower 

 layer; d, cells of the cicatricula from an egg in its 

 descent through the oviduct in process of formation. 

 A scale with divisions of ^ of an inch is appended. 



spherical ; as, for example, in the cartilaginous 

 fishes, in which a remarkable variety occurs 

 of a cubical form, and sometimes these mixed 

 with tetrahedral forms, as in the skate.* 



When free, these corpuscles in the yolk of 

 the bird's egg roll easily on the surface of a 

 plate of glass as perfectly distinct spherical bo- 

 dies. They present (see jig. 52, A) a minutely 

 molecular or granular aspect, but with quite a 

 smooth outline or margin to the whole cor- 

 puscle. If subjected to pressure, or cautiously 

 ruptured with needle points, they break 

 readily at one or more places, and allow the 

 escape from their interior of the thick granular 

 fluid, which flows slowly out of them in a 

 stream. The granules are in large quantity, 

 as compared with the fluid in which they are 

 suspended, and are most of them of an ex- 

 tremely minute size, probably below -5^0-0 f 

 an inch in diameter. 



* See Miillcr's Physiology, vol. ii. 



Although the yolk corpuscles present the 

 distinct external margin now mentioned, and 

 thus constitute capsules containing the gra- 

 nular fluid, yet we cannot, in most instances, 

 detect any vesicular membranous envelope 

 surrounding them. One may sometimes ob- 

 serve a delicate limiting line ; but it has been 

 impossible for me to determine whether it 

 consisted really of a membrane or of a thin 

 condensed layer of the granular substance or 

 plasma containing it. At an earlier period it 

 is probable that these corpuscles have mem- 

 branous envelopes, but when fully formed the 

 greater number are certainly destitute of 

 them ; for occasionally a larger corpuscle may 

 be observed to divide into smaller ones, the 

 outlines of which are nearly as distinct as 

 that of the larger corpuscles. 



Nor is any nucleus in general to be per- 

 ceived in these corpuscles. I have occasion- 

 ally seen in those from which the granular 

 matter was escaping, and which had thus be- 

 come less opaque than usual, a slight ap- 

 pearance of a clear hyaline circular space, but 

 it scarcely deserved the name of nucleus ; and 

 if these spherical bodies are to be regarded as 

 cells, which 1 think they ought, it must be in 

 a somewhat different acceptation from that in 

 which the term cell has hitherto been gene- 

 rally applied to vesicular organised structures. 

 But recent researches on the early condition 

 of cells seem to have rendered it necessary 

 that we should include under this denomina- 

 tion several simple spherical minute forms of 

 organised or organising matter, which were 

 not at first regarded as true cells by the 

 authors of the cellular view of organic struc- 

 ture ; and when we consider the mode of their 

 formation, it is more than probable that the 

 vitelline corpuscles now under consideration 

 may be included among the number.* 



They probably constitute, at all events, as 

 Schwann has first shown, one stage of deve- 

 lopment of a cellular structure ; and, in the 

 meantime, they may with propriety be styled 

 the larger granular yolk corpuscles. 



There is considerable uniformity in the ap- 

 pearance and structure of these corpuscles in 

 nearly the whole of the deeper-coloured por- 

 tion of the yolk ; but immediately below the 

 vitelline membrane, several layers of them 

 are of a smaller size, and the outermost layer 

 of all consists of cells which are much smaller 

 and more compressed, distinctly nucleated and 

 of a short cylindrical or prismatic shape. In 

 some places also, corresponding to the con- 

 centric lighter lines which run through the 

 yellow yolk, some approach is seen to the 

 next kind of yolk cells or corpuscles, which I 

 shall have to describe viz., those of the cavity. 



The substance of the yolk-cavity and canal, 

 which in the unboiled egg may be distin- 

 guished from the other part by its lighter 



* The above observation has a general application 

 to such minute spherical masses of matter as are 

 destitute of external envelope or nucleus : but in 

 rc'li-rence to the corpuscles of the yolk, I ought to 

 observe that Schwann regarded them as cells in 

 various stages of growth. 



