Observations on the development of Agelena naevia / by William A. Locy.

  • Locy, William A. (William Albert), 1857-1924.
Date:
1886
Catalogue details

Licence: Public Domain Mark

Credit: Observations on the development of Agelena naevia / by William A. Locy. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    In freshly laid eggs this layer is in contact with the vitelline mem- brane, but early becomes separated from it by the contraction of the vitellus. The perivitelline fluid which makes its appearance during this process is coagulable by heat and is also stainable. At first the contrac- tion of the egg is uniform on all sides, but soon it takes place more rapidly on one side, thus giving rise to a flattened surface (compare Fig. 5), upon which the ventral plate is afterwards established. Through the pressure of this contraction the blastema is moulded upon the periph- eral yolk corpuscles into regions that correspond in position and size with the underlying corpuscles. Owing to mutual pressure these regions become regular hexagonal areas, (Pl. Y. figs. 24, 25,) and resemble the subsequently formed cells of the blastoderm. The absence of nuclei is the fundamental feature that at once distinguishes them from the blasto- dermic cells, though they have frequently been mistaken for such on the supposition that the nuclei were obscured. The division of the blastema into areas as described above is a very early phenomenon. At the time of my first observations a number of faintly marked areas had already made their appearance at the more active (animal) pole. At this time they could not be detected upon the opposite hemisphere; but after a short interval they also made their appearance there in isolated patches; finally they covered the entire sur- face of the egg. At the outset the boundary lines of the areas are very faint, but they become more distinct as the contraction of the vitellus continues. In some places the yolk corpuscles become separated from the blastema by a more rapid contraction of the interior protoplasm, and then the polygonal areas in such regions remain only partially outlined and incomplete, as described and figured by Balbiani (’73, Fig. 2). After the areas are definitely formed the yolk corpuscles sometimes shift their original positions, and thus cease to coincide with the areas, since the latter do not at the same time undergo corresponding changes. The next alteration in the surface makes its appearance only after the lapse of a considerable interval (twelve to forty-eight hours) ; this led Balbiani to assert, erroneously, that the egg is undergoing a period of rest. Sections show on the contrary, that the interim is one of great internal activity, during which repeated divisions of the nuclear sub- stance lead to the formation of numerous cells which migrate towards the periphery. The appearance of some of these at the surface marks the beginning of new surface changes. The cells thus emerging from the yolk constitute the primary blastoderm ; they first appear in the interspaces between the yolk corpuscles, but often migrate afterwards to