During the non-breeding season the lobules of the mammary glands consist of solid cords of cells which may occasionally be embedded in a pad of fat tissue.
During the breeding season some mammary glands can exhibit a tubular grade of organization and can even secrete milk without the intervention of pregnancy. Apparently the stimulus of pregnancy and incubation of an egg in the pouch is generally, but not invariably necessary for development of an alveolar grade of organization. The ducts leading from the lobules to the skin of the areola undergo hypertrophy during the course of lactation.
The present work shows that the gestation period is at least 16 days long but evidence in the literature shows it may be as long as 27 days. The incubation period of an egg in the pouch was found to be of the order of 10–10·5 days.
A newly-hatched echidna weighed 378 mg and was 1·47 cm long; it exhibited an egg tooth and the remains of foetal membranes attached to the abdomen. The newly-hatched had well-developed olfactory organs including sensory cells in the olfactory epithelium. The alimentary tract exhibited a degree of differentiation consistent with the notion that it could digest milk.
The fatty acid composition of the triglyceride fraction of a sample of milk taken just after hatching differed from that of mature milk in that the ratio of the concentration of oleic acid to palmitic acid in the early milk was about unity whereas it was 3·7 in mature milk. A high level of arachidonic acid may be a characteristic of the milk at hatching.
Iron was present in a concentration of 8·3 μg/ml in the early milk, and of 43·0 μ/mlg/ml in a sample of mature milk. The regression of the mammary glands from the alveolar to the solid-cord state is described.
Two corpora lutea of pregnancy, taken about 11 and 19 days after egg laying, were degenerate.