Annulate lamellae have been observed in the myocardium of 18-day-old chick embryos maintained at the normal temperature of 100°F and at 90°F during the last week of incubation. An increased number of annulate lamellae was observed in heart muscle of embryos incubated at 90°F. This is probably caused by a persistent production of these organelles, since annulate lamellae are present in greater frequency than in 11-day-old embryos incubated at 100°F. In the hypertrophic hearts of 18-day-old embryos incubated at 90°F, the annulate lamellae were associated with a net increase of protein content and an elevated concentration of myocardial glycogen. It is suggested that the increased number of annulate lamellae is a sequela of reduced environmental temperature during incubation.

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