Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/79

Rh applied to a drop of the contents of the egg, it does not appear to act materially upon the cells, and the contained corpuscle becomes paler and somewhat swollen, which could not well take place if it were fat. These cells, then, are the earlier stage of development of the subsequent globules of the yelk-cavity. The larger ones already resemble them perfectly. These globules of the yelk-cavity are therefore likewise cells. The nucleus-globule (Kernkugel) is acted on by acetic acid precisely in the same way as it was in the earlier condition. It does not lie centrally in the cell, but on the internal surface of the wall, as is seen when the cells are caused to roll under the microscope. When at rest, however, they are generally so placed that the nucleus-globule occupies the most depending point (because probably it is the heaviest portion of the cell), and, on that account, it then appears to lie in the centre of the cell. The yelk in the first instance contains only the yelk-cavity, with its cells; the proper yelk-substance with its globules not being as yet formed. The colour of these young eggs is therefore also white, like the contents of the yelk-cavity.

The membrane-like layer which surrounds the above-described contents of the egg, may be completely separated from the parts which surround it externally with facility, after the egg has been divided through the centre. It is not connected with them, and appears, to the unaided eye at least, to be pretty smooth on its external surface; it is not possible to trace it towards the interior. Its structure is peculiar. Purkinje, who discovered it, describes it as consisting of globules, which resemble in form and size, but are more transparent than the blood-corpuscles. When spread out upon a plate of glass, and examined with the microscope, it is seen to consist of two parts, an internal minutely granulous stratum, and an external layer of cells. Numerous little granules are observed in the internal stratum, which resemble the nuclei of the above-described cells of the yelk-cavity in their earliest stage, and I conjecture that the cells of the yelk-cavity are formed from this stratum, so that in fact it still pertains to the yelk-cavity. The external layer consists of small round granulous cells, each of which contains a nucleus, which again in many instances encloses one or two nucleoli. Two or three such layers of cells lie one above another, These layers of cells are surrounded externally