1. In Sponges, e.g. Grantia (Gatenby), some Annelids, e.g. Glycera (Retzius), Coelentrates, e.g. Alcyonium (Retzius), some Molluscs, e.g. Chit on (Retzius), Amphioxus (Retzius), Nemerteans, Echinoderms, and Fishes, e. g. Amia (Retzius), the unchanged mitochondria remain clumped together to form a ‘nebenkern’ at the base of the nucleus of the ripe sperm.

2. In Mammals (Meves, Regaud, Duesberg, Retzius, and Gatenby and Woodger) the mitochondria form the sheath of the ‘middle-piece’. An essentially similar condition prevails in Birds, e.g. Gallus (Retzius), and Reptiles, e. g. Testudo (Retzius).

3. In Saccocirrus, Paludina, Limax (Gatenby), and Scorpions (Wilson, Gatenby and Bhattacharya, and Nath) the mitochondria form the sheath of almost the whole length of the axial filament.

4. In the highly specialized case of Insects, e. g. Aptera, Orthoptera, Lepidoptera, Coleoptera, and Hemiptera (Bowen), the mitochondrial ‘nebenkern’ undergoes a progressive condensation, and the sheath of the axial filament is a new structure altogether.

Original Observations.

1. In Oporabia, Smerinthus, and Pieris the mitochondrial ‘nebenkern’ does not directly form the tail-sheath, but undergoes a progressive dwindling and ultimately disappears. The sheath-forming substance arises as a new substance in the chromophobic cavity round the ‘nebenkern’.

2. The structure of the Lepidopteran mitochondrial ‘nebenkern’ is alveolar or ‘plate work’ and not a ‘spireme’.

3. In the centipedes (Lithobius forficatus) the mitochondria form the tail-sheath directly.

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