Abstract The following paper constitutes the first instalment of an account of the development of Lepidosiren. The material upon which it is based was obtained during a sojourn which I made for the purpose amongst the swamps of the Gran Chacot during the years 1896 and 1897. I should in some ways have preferred not to publish until in a position to give a fairly complete and rounded off account of the whole of the phenomena of development. This would, however, have meant very considerable delay, and I am induced to publish the set of drawings illustrative of the external features, both by the advice of my friends and by the fact that this will serve to make known those features in the development which are after all of the greatest general interest. It will be time enough when the research is completed to point out any conclusions that may be drawn from it as to the affinities of the group to which Lepidosiren belongs; but I may be allowed even now to draw attention to one point which seems to me to lend unusual importance to the features attending its development. That is the peculiar conditions under which the development will be found to take place—in a secluded retreat, in which, while able to hatch at a comparatively early stage, the larva continues to exist, performing only those vital processes necessary to all life, sheltered effectually from having to fight for itself amongst those ever recurring changes in environment, which bring in their train the hiding away under a veneer of adaptation of features due to long heredity and of phylogenetic importance. The development of Lepidosiren under such conditions of seclusion—conditions, too, which probably remain constant over very long periods of time—is enough, I think, to lend a very special interest to its developmental phenomena, quite apart from the interest they must have as belonging to one of the Dipnoi and to the first of the Dipneumona which has fallen under embryological investigation.