Ultrastructural studies on adult Nippostrongylus brasiliensis removed at varying stages after infection from rats given a single dose of 2000 infective larvae have shown that changes in their cytology occur and that these can be correlated with the onset and development of immunity to this nematode in the host. The most noticeable changes are resorption of spermatozoa by the male reproductive tract, a drop in egg production, changes in the cytology of the intestinal cells, and the appearance of many large droplets, which look like droplets of lipid, in the cytoplasm of the hypodermis, muscles, excretory glands, reproductive system and the intestine. There are no obvious changes in the structure of the cuticle of the nematode during the course of infection. Possible reasons for the onset of these changes in the nematode are discussed.Much of this work was carried out during the tenure of grants from the U.S. Public Health Service (grant number AI 04275) and from the Agricultural Research Council while I was working at the Molteno Institute in Cambridge. I wish to thank the late Professor J. D. Boyd and Dr C. C. D. Shute of the Department of Anatomy, Cambridge, for permission to use the electron microscopes, Mrs B. Fisher for technical assistance and Mr Paul Rogers for assistance with the photography.

The structure, method of feeding and relationship with the host cells of the tissue parasitizing stages ofHistomonas meleagridishave been studied by means of light microscopy, cytochemistry and electron microscopy. The invasive stage is amoeboid, lacks flagella but has what appears to be remnants of the flagella apparatus, and feeds by phagocytosis. The vegetative stage is a round quiescent form. It apparently feeds by secreting proteolytic enzymes, which carry out extra-corporeal digestion of the host's tissues, and then taking up small particles of disrupted host cells by means of pinocytosis and probably by diffusion. The vegetative stage also contains remnants of flagella apparatus. Neither the invasive stage nor the vegetative stage contains mitochondria and succinic dehydrogenase is absent. It is concluded that there is no resistant stage.The reaction of the host's tissues to the parasites has been briefly described.The authors are grateful to Dr C. C. D. Shute for permission to use the electron microscope, to Mrs B. Fisher and Mr M. Shirley for technical assistance and to Mr P. Rogers for assistance with the photography.

The ultrastructure of the normal caecum of chickens and of chickens infected withTrichomonas gallinarumhas been investigated. In infected caeca some villi and crypts of LieberkÜhn appear normal whereas others show marked changes in structure of the epithelial cells. The most obvious changes are loss of microvilli and glycocalyx and an increase in number of vesicles which stain metachromatically with Azur II.T. gallinarumhas been detected between epithelial cells and in the lamina propria. Bacteria often, but not always, accompanied the protozoa in the tissues.

SUMMARYThe total lipids and the amount of neutral lipids of male and female N. brasiliensis, from 7-day and 15-day infections, have been studied quantitatively. There is an increase in total lipids from 10·9 ± 2 to 15·5 ± 2 % in females and an increase from 8·4 ± 1 to 14·6 ± 2 % in males. In both sexes there is a rise in cholesterol and trigylceride content and a fall in free fatty acid content. These changes are discussed with reference to the development of immunity of the host to this nematode.I wish to thank Mrs B. Fisher, Mr A. W. Pearson and Mr N. Sykes for assistance during this work. These results form part of a programme of work supported by the A.R.C. while I was at the Molteno Institute, University of Cambridge, and continued at the Houghton Poultry Research Station. I wish to thank Dr P. Tate for accommodation at the Molteno Institute during the early part of this work.

The ultrastructure and development ofHistomonas meleagridisin the reproductive tract of maleHeterakis gallinarumhas been described. In the testis the protozoon is a relatively large amoeboid organism which lies between the epithelial cells of the testis and the spermatogonia, spermatocytes and spermatids. It is similar in appearance to the invasive stage which is found in the final host and like the invasive stage, it also feeds upon the host cells by phagocytosis. In the vas deferens the protozoon lies between the host cells, where it is much smaller than the stages in the testis but still amoeboid, and inside the cells of the vas deferens.Intracellular stages are much smaller than the other stages, are rounded in appearance and cause little or no damage to the host cell. They are similar to the stages found in oocytes of the female nematode. The possible role of the male nematode in the life cycle of the protozoon is discussed.I wish to thank Mrs B. Fisher for technical assistance and Mr B. Carter for assistance with the photography.