In order to study the effects of paralysis on the development of the skeletal system in the chick embryo, embryos were paralysed by periodical administration of decamethonium bromide at 6, 8 and 10 days of incubation and were compared with controls to which saline was administered. 1. Experimental embryos showed retardation of the developmental stages of Hamburger & Hamilton (1951) and were of smaller size and weight. 2. Whole mount clearing showed that experimental embryos had scoliosis, cartilaginous or even bony fusion of cervical vertebrae, the knee joint and some other joints, distortion of the pubis and scapula, and failure of chondrification of the patella and the plantar tarsal sesamoid bone. 3. Linear measurement showed reduction in length of long bones, e.g. femur, tibia, humerus, ulnar and radius and lower and upper beaks in experimental embryos. The upper beak showed a greater reduction than the lower. 4. Volumetric measurement of the tibia showed a reduction in all tissues in experimental embryos and a marked reduction in cartilage formation and resorption between the 13th and 14th days of incubation. 5. Timings of onset of invasion by the osteogenic bud, of ossification assessed histologically and of calcification assessed by alizarin staining were similar between control and experimental embryos. 6. Histological and scanning electron microscopical study of the tibia revealed no changes between the two groups until the 11th day of incubation. From 12 days onwards:- i) less periosteal bone formation occurred in the experimental embryos, ii) cartilage resorption was not progressing in the experimental embryos at the extent of their corresponding controls iii) blood vessels in the Haversian canals were larger in diameter in the experimental embryos, iv) the tibiofibular interosseous ligament and fibular crest of the tibia regressed in experimental embryos. 7. Muscle fibres were atrophied and lost their orderly arrangement in the experimental embryos
