The Development of the Ovary and Ovarian Egg of a Mosquito, Anopheles Maculipennis, Meig

Abstract

1. The period at which the ovaries of A. maeulipennis commence to develop depends on the season and locality. Normally this is from about the middle of March to the beginning of April. 2. A meal of blood appears to be necessary for the production of eggs. 3. One meal of blood is sufficient to cause eggs to be produced. After the lapse of a day the large blood-mass in the stomach shows two zones: a posterior partially-digested portion and an anterior portion of apparently fresh blood. This appearance has sometimes been taken as evidence that more than one meal of blood has been consumed. 4. The eggs are fully developed six days after the insect has fed on blood. 5. In the case of two insects which were observed at the time of oviposition the eggs were laid immediately after dark. 6. The muscle-bands of the ovarian sheaths are striped; not unstriped as is usual in insects. 7. A large number of vacuolated cells are found in the ovary. The nature of these is not clear, but they appear to have some relation to the tracheal system. 8. The chorion of the egg is highly specialized to retain air round the egg, and the buoyancy of the egg is entirely due to the entrapped air. 9. The floats are-produced by a very specialized form of folding of the follicular epithelium. 10. The micropyle apparatus is produced by specialized cells of the epithelium, the ‘rosette-cells’. 11. Immediately below the micropyle is a specialized portion of the inner wall, the ‘stopper’. 12. The inner and outer walls of the egg, though formed of entirely different substances, are both secreted by the follicular epithelium. 13. The inner wall is first gelatinous in nature and transparent; but, after the egg is laid, becomes brittle and dark in colour, causing the egg to appear black. This change in character only takes place when the eggs are laid on water. 14. The mature egg contains two distinct kinds of yolk, one of large granules which are proteid in nature, and the other of small granules the nature of which I have been unable to determine. 15. There are two distinct periods of growth of the eggfollicles, the first culminating in the ‘resting stage’ and the second only commencing after the mosquito has bad a meal of blood. 16. Each egg-follicle consists of a follicular epithelium surrounding seven nurse-cells and an oocyte. These appear to be the product of a single oogoniuni. 17. The cells of the follicular epithelium multiply by mitotic division during the whole of the first period of growth. In the second period, though the follicular epithelium increases greatly in area, this is due purely to the increase in size of the individual cells. 18. From the earliest stages separate vegetative and germinal portions can be distinguished in the oocyte nucleus. 19. During the second period of growth the oocyte nucleus branches throughout the entire oocyte. 20. The branching nucleus, in conjunction with the nursecells, takes an active part in the nutrition of the oocyte. 21. The branching of the nucleus may be regarded as a mechanism for the purpose of increasing the surface. 22. I have observed a similar method of branching of the oocyte nucleus in Theobaldia annulata, and it probably also exists in Necrophorus vespillo and Si1pha sp. 23. The branching nucleus is almost entirely the product of the nucleolus. 24. The branching nucleus is morphologically and physiologically comparable to the Corpuscles of Balbiani of other animals. 25. The germinal portion of the nucleus, the ‘chromatin residue’, is the product of the condensation of the spirerae threads. 26. The ‘chromatin residue’ becomes invisible for a short period and reappears after oviposition as the segmentation nucleus. 27. The chromatin of the active nurse-cells consists of minute bars situated on a much convoluted band of linin. 28. Degeneration of a certain number of egg-follicles is normal during the development of the ovary. 29. Sporozoa are frequently found in the eggs, often completely replacing the whole of the yolk.