The developmental stages of Oreochromis niloticus are similar to those described in other mouthbreeding tilapias except that, as in zebrafish, no cavity was found in the blastula. Variation in the rate of development of the embryo and larva of O. niloticus was found within a clutch of eggs as well as between clutches. Hatching glands are described for the first time in tilapias. They are widely distributed within the ectoderm covering the head, body, tail, and surface of the yolk sac near its attachment to the embryo. Timing of larval development is similar to that in other mouthbrooding tilapias, but is slower than that found in substrate-spawning tilapias. A pneumatic duct connects the swimbladder to the digestive tract and swimbladder inflation and initiation of feeding occurs at about the same time. The digestive tract of the larva 8 and 9 days after fertilization is similar to that found in the adult, except that there are no digestive glands. An endocrine pancreatic islet was first seen 76 h after fertilization. A prominent thymus gland is present at 100 h. Hematopoietic tissue develops in the vicinity of the pronephros during early larval development. A spleen develops later, 7 days after fertilization.

MonnIsox, c. M., eNo P. H. oopNsp. 1978. Distribution and morphology of the rodlet cell in fish. J. Fish. Res. Board Can. 35: 101-116.A short review of the work done on the rodlet cell is presented. The morphology and distribution of rodlet cells in several species of fish, primarily marine, is described and compared with that reported by other authors, mainly in freshwater fish. The morphology in freshwater and marine fish is similar, but variations are present in different tissues ln the same fish. The rodlet cell was found in a wide range of tissues in most of the fish studied. It usually occurs in epithelia, with the apex facing the lumen. The possible function of the rodlet cell is discussed. MonmsoN, c. M., nNn P. H. oopNsr. 1978. Distribution and morphology of the rodlet cell in fish. J. Fish. Res. Board Can. 35: 101-116.Les auteurs passent bridvement en revue les travaux eftectu6s sur les bitonnets. Ils d6crivent la morphologie et la distribution des bAtonnets chez plusieurs espdces de poissons, surtout de mer. Ils les comparent avec les observations d'autres auteurs, en grande partie sur des poissons d'eau douce. La morphologie est semblable chez les poissons d'eau douce et les poissons de mer, mais il existe des variations dans diff6rents tissus d'un m6me poisson. On trouve des bdtonnets dans une gamme 6tendue de tissus chez la plupart des poissons 6tudi6s. La cellule se rencontre ordinairement dans l'6pith61ium, l'apex donnanr sur la lumidre. On analyse la fonction possible de la cellule en bitonnet.

The digestive tract of Oreochromis niloticus is described, in order to resolve discrepancies found between previous accounts. Two types of goblet cells were found in the oesophagus, which differed in size, and in staining characteristics with periodic acid-Schiff and alcian blue. A region with tubular glands consisting of large mucous cells was continuous from the entry of the oesophagus, across the anterior part of the stomach to the pyloric valve, essentially providing a bypass circumventing the sac-like portion of the stomach. This region, which is lined with striated muscle, may be a means of disposing of unwanted material, either by regurgitating it, or by passing it rapidly along to the intestine. Large mucus cells in the tubular glands and the neck cells of the gastric glands probably protect the mucosa from the very acid contents of the stomach. An ileo-rectal valve was present. There is therefore a separate intestine and rectum in O. niloticus, as in most teleosts. 1999 The Fisheries Society of the British Isles