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

  • William Albert Locy
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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    The spinning glands are not yet definitely established, but in the anal region on the ventral side of the proctodasum there is a large accumula- tion of ectodermic cells (PI. XI. fig. 70) from which they are subse- quently developed. Late in this period the infoldings for the lungs arise. There appear a pair of large oval masses of cells, the nuclei of which are arranged in par- allel lines (PI. XI. fig. 73). From these cells the respiratory lamellm of the lungs are finally formed. The mesoderm likewise has been growing rapidly during reversion. In the previous period it was confined to the ventral portion of the em- bryo, but during the present period it grows upward on either side until it reaches the dorsal median line, thus forming a continuous layer be- neath the ectoderm, as well as an investment for all organs, which arise as outgrowths of either ectoderm or entoderm. The dorsal growth of the rudimentary terga, already spoken of as external features, is followed- by this underlying layer of mesoderm. Early in the formation of the dorsal elements this mesodermic layer is divided into corresponding somites. Balfour (’80, p. 181) concluded that the cells out of which are formed the dorsal somites of the meso- derm “ are not derived from prolongations of the somatic and splanchnic layers of the already formed [ventral] somites, but are new formations derived from the yolk.” My sections, however, indicate that there is a direct continuity between the two (PI. IX. figs. 59, 61), and that the dorsal mesoderm is an outgrowth from the previously established ventral mesodermic somites. It is during this period also that the heart is formed. While I have been unable to arrive at an entirely satisfactory understanding of the details of its formation, I am convinced that it is not, as Balfour states, developed from a solid cord of cells, but from the dorsal limb of the up- growing mesoderm, and that its dorsal wall is closed first, while the ventral wall — the floor — remains for a time widely open below, thus communicating freely with the yolk. My sections also show that at a later period the aorta is formed, by means of a constriction, from the mesenteron. This agrees with the recent observations of Schimkewitsch, (’84a). A layer of characteristic cells, to which Balfour alludes in speaking of the formation of the dorsal mesoblast, precedes the formation of the heart in the dorsal region. These are what; have been called “ primary entoderm ” cells, and are sharply distinguished from the surrounding cells by their large size, their large, oval nuclei, and their yellowish tint. These VOL. xii. — no. 3. 6
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