Page:Proceedings of the Royal Society of London Vol 4.djvu/163

Rh form before any other part. The central portion of the nervous system sometimes attains a considerable degree of developement, although it be exceedingly minute ; thus an instance has been met with in which the developement of this part had reached a stage scarcely inferior to that in another instance, in which the corre- sponding part measured more than ten times the length.

There does not occur in the mammiferous ovum any such pheno- menon as the "splitting" of a membrane into the so-called "se- rous, vascular, and mucous laminae." Rathke had already found that parts previously supposed by Baer and others to be formed by the so-called "germinal membrane," really originate independently of it : these parts are the ribs, pelvic bones, and the muscles of the thorax and abdomen, which according to Rathke arise in a part pro- ceeding out of the " primitive trace" itself. Reichert had previously discovered that the part originating the lower jaw and hyoid bone " grows out of the primitive trace." The author beginning with an earlier period goes farther than these observers, and shows that the so-called " primitive trace" it«elf does not arise in the substance of a membrane, but presents a comparatively advanced stage of the object above described as the true germ. Hence the author suggests, there is no structure entitled to be denominated the " germinal membrane."

The most important of the foregoing facts respecting the deve- lopement of the mammiferous ovum, however opposed they may be to received opinions, are in accordance with, and may even explain, many observations which have been made on the developement of other animals as recorded in the delineations of preceding observers. If in the ovum of the bird the germinal vesicle in like manner re- turns to the centre of the yelk, the canal and cavity known to exist in the yelk of that ovum might be thus explained. The ovum may pass through at least one -and- twenty stages of developement, and contain, besides the embryo, four membranes, one of which has two laminse, before it has itself attained the diameter of half a line, a fifth membrane having disappeared by liquefaction within the ovum.

The size of the minute ovum in the Fallopian tube and uterus affords no criterion of the degree of its developement ; nor do any two parts of the minute ovum, in their developement, necessarily keep pace with one another.

The proportion of ova met with in these researches, which seemed to be abortive, has amounted to nearly one in. eight. Sometimes two yelk-balls exist in the same ovum. With slight pressure, the ovum, originally globular, becomes elliptical. Its tendency to as- sume the latter form exists especially in the chorion, and seems to be in proportion to its size.

The author has discovered that when the germinal vesicle is first seen it is closely invested by an extremely delicate membrane. This membrane subsequently expanding is that in which the yelk is formed. He has traced the chorion from stage to stage up to the period when it becomes villous, and shows that it is not, as he formerly supposed, the thick transparent membrane itself of the ovarian