The Formation and Structure of the Chorion of the Egg in an Hemipteran, Rhodnius prolixus

Abstract

ABSTRACT The main regions of the Rhodnius prolixus egg-shell have been defined; a concise definition has been obtained for the term ‘chorion’. The formation and structure of the unspecialized chorion has been followed from the time of differentiation of the follicle cells, up to the completion of the shell, and an assessment made of the chemistry and permeability of each component shell layer. The follicle cells are binucleate ; changes in morphology and histology prior to secretion of the shell are outlined. The secretory products of the follicle cells fall naturally into an endoehorion and an exochorion; the endoehorion consists of five modifications of a proteinaceous substance. They are, in order of secretion: The Inner Polyphenol Layer, which consists of a series of tanned granules of average diameter 2 μ, containing large quantities of polyphenols. The layer is discontinuous and has no effect on permeability. The Resistant Protein Layer, a tanned and possibly vulcanized layer of protein, 1 to 2 μ thick, containing diffuse polyphenols. It is resistant to strong acids and bases, and permeable to water, ions, and large water-soluble molecules. The Outer Polyphenol Layer, which is similar to the inner layer, but has more minute granules. The Amber Layer.—This is the only coloured layer of the shell, and is less than 0-1 thick. It consists of tanned protein to which oil is added after secretion. It is therefore a lipidized protein, which is excessively resistant to acids and alkalies, and permeable to oils and oil-soluble material and to small ions and water. 5. The Soft Protein Layer.—This is a thick laminated layer some 8 ft thick, similar to, but less resistant than, the resistant protein layer. It contains polyphenols and tyrosine. The layer is freely permeable to -water-soluble substances. Throughout the secretion of the endochorion, the follicle cells stain deeply and appear to be filled with the protein components of the shell. The exochorion consists of two layers of the lipoprotein ‘chorionin ‘. 6. The Soft Exochorion Layer is a lipoprotein which is soluble in potash but not in strong acids ; the layer is permeable to lipoid solvents and to water and small ions, but not to larger particles. It is 8 p. thick at its maximum thickness, but contains follicular pits which, during secretion, are filled by long processes from the follicle cells. 7. The Resistant Exochorion Layer is a more resistant form of chorionin. It lines the pits and covers the surface of the shell, giving rise to the polygonal markings corresponding to the follicle cells, each with a pit at its centre. The follicle cells contain quantities of lipoprotein during this phase of secretion, and are difficult to stain. A method of staining is described which shows that pore canals of two varieties are present in the exochorion layers only. They run from the walls of the pits but do not reach the endochorion. None of the layers of the chorion waterproofs the shell. In the rear end of the shell, the outer polyphenol layer is displaced towards the exochorion, thus increasing the resistant protein layer and reducing the soft protein layer. It is shown that all seven layers are present in the neck, and in the central region of the cap, and that the order of secretion is the same. Modifications are produced by variations in the thickness of the various layers. In the neck, the soft protein layer is reduced ; in the cap, the resistant protein layer is reduced while the amber layer is 2/z. thick, giving the cap a brown appearance. The soft protein layer is extremely thin and irregular while the exochorion layers are 16/z, thick. Pore canals are again present in two varieties. Some analysis is made of the formation of follicular pits; this appears to be correlated with the thickness of the exochorion and endochorion layers.