Bauhaus-Universität Weimar

ITS STRUCTURE. 
823 
germinal spot consists of one or more somewhat opaque corpuscles, and is possibly 
the analogue of the nucleus of formative cells. It is simple in the germinal vesicle 
of the human subject, of Mammalia, birds, reptiles, and many Invertebrata, and is 
recognisable even in the ova which are least advanced in development. In Amphibia, 
the osseous fishes, and several invertebrate animals, the germinal spot is multiple. 
As the ova advance in development granular matter is deposited on the inner surface 
of the germinal vesicle, and renders the germinal spot or spots indistinct, and some¬ 
times even invisible. In some of the Invertebrata the germinal spot itself seemed to 
Wagner to have a special investing membrane. 
The ova of mammalia and the human subject find the nutriment necessary for the 
development of the embryo in the uterus. Hence the extremely small quantity of 
yolk with which they leave the ovary, and their minute size; their diameter, in their 
most perfect condition, being scarcely so much as one-tenth of a line. In their rela¬ 
tion to the ovary, also, they differ in many particulars from the ova of oviparous 
animals. Owing to their extreme minuteness, the ovula of man and mammiferous 
animals for a long time escaped observation. Prévost and Dumas had remarked, that 
the ova found in the oviducts of animals, shortly after impregnation had taken place, 
were much smaller than the follicles of Graaf; and in two instances they actually 
saw the ovulum within the Graafian follicle, but they did not pursue the subject any 
farther. The merit of discovering the ovulum of mammiferous animals and man 
really belongs to Von Baer. 
The vesicles or follicles of Graaf, or the ovicapsules which contain the ovula of 
mammals and the human subject, are connected together by a very firm stroma, 
which, with them, constitutes the ovary. They are but slightly prominent above the 
surface of the ovary in most Mammalia, but in the Ornithorynchus are raised on pedi¬ 
cles as in birds. Each capsule is formed of two membranes, the more internal of 
which is like the ovicapsule of oviparous animals, lined with epithelium (membrana 
granulosa of Baer). The ovulum occupies only a very small part of the‘cavity of the 
Graafian vesicle or capsule, the remainder being filled with an albuminous fluid in 
which microscopic granules float. In immature capsules the ovulum is proportion¬ 
ally larger and lies more nearly in the centre; while in those which are fully formed 
it is placed close to the inner wrall of the capsule, embedded in a granular zone. In 
both conditions, however, according to Barry, the ovulum is attached to the parietes 
of the follicle by peculiar granular bands or retinacula. In order to examine an ovu¬ 
lum, one of the Graafian vesicles, it matters not whether it be of small size or arrived 
at maturity, should be pricked and the contained fluid received upon a piece of glass. 
The ovulum then being found in the midst of the fluid, by means of a simple lens, 
should be placed under the compound microscope. Owing to its globular form, how¬ 
ever, its structure cannot be seen until it is subjected to gentle pressure under a 
second thin lamina of glass, or by means of a compressorium. 
The external investment of the ovulum is a thick membrane, which, under the 
microscope appears as a bright ring bounded externally and internally by a dark out¬ 
line. This membrane is called by Valentin and Bernhardt, zona pellucida; by R. 
W'agner, chorion. The anatomists who have occupied themselves most with the ex¬ 
amination of this part of the ovulum, are divided in their opinions respecting its 
nature. According to Krause, it is composed of an albuminous mass, enclosed in a 
delicate membrane; while W'agner and Bischoff regard it as a simple tunic, because 
it tears with a uniform margin. Schwann admits that such is the character of its 
edge when torn, but .nevertheless inclines, as does Dr. Barry* also, to Krause’s 
opinion. 
Within this transparent investment lies the yolk, which consists of granules or cells 
and fat globules. This substance forms a globular mass wrhich is usually in close 
contact with the inner surface of the investment above described. But sometimes in 
ova, which have attained the most perfect maturity, an interval can be seen between 
the yolk and its outer tunic, and this interval is rendered greater by the imbibition of 
* According to Dr. Barry, the zona pellucida is a solid transparent membrane. He states, 
however, that internal to this there is a much thinner membrane (the true membrana vitelli), 
which exists while the ovum is contained in the ovary but disappears by liquefaction in the 
Fallopian tube, at the same time that the chorion begins to be formed. See Dr. Barry’s Re¬ 
searches on Embryology, 2d series, p. 332, 333, and 339.
        

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