One hundred and forty-three skulls of a total of 15 cetacean species belonging to the superfamiIies of the Platanistoidea, Monodontoidea and Delphinoidea presented usually symmetrically arranged perforation areas or lacunae in the neurocranium. A minimum of 1-2 skull bones are affected (Delphinapterus, Monodon, Platanista, Cephalorhynchus), with a maximum of seven in Phocoenoides. The bones most commonly showing fenestrations are the exoccipitale (14 species) and the supraoccipitale (12 species), followed in descending order by the basioccipitale (7 species), the frontale and vomer (6 species), the parietale and basisphenoid (3 species) and lastly the maxilIare (only one species).
Fenestration in the neurocranium occurs in both male and female animals of very different ages. It does not, however, affect all the individuals in a particular species.
The perforation areas occur most often in the continuity of a bone, and occasionally close to the sutures. They are persisting structures, already present in the foetus, which are not comparable with fontanelles in terms of either position or structure, and do not occur in the region of the emissary veins. Both membrane bones and parts deriving from the primordial cranium are affected by the fenestration. From the standpoint of embryology the lacunae are to be seen as primary aplastic zones of the skull in which formation of bony tissue has come to a halt.
Three main forms of fenestration may be distinguished:
1. Greatly attenuated, transparent but stilI intact bone blates.
2. Greatly attenuated bone plates interspersed with pores and fissures.
3. Openings in the skull wall ranging from several milIimetres to a number of centimetres in size.
The fenestrations are of different shapes and sizes in the various species, appearing sometimes with frayed edges and sometimes as smooth edged, oval to round openings.
The highest degree of fenestration is to be found in Phocoenoides truei and, occasionally, in Pontoporia blainvillei. In the intact skull the lacunae are always closed by the periosteal membrane.
Histological examination of a foetal specimen of Delphinapterus and a subadult specimen of Neophocaena revealed no absorption of preformed bony tissue in the region of the lacunae. The fenestration appears as a primary, aplastic zone, in which bone formation has ceased. The spongy part of the osseous tissue forms a wedge together with the osteogenetic layer of the periosteum which is still present at this stage of development. Only the fibrous parts of the outer and inner periosteum unite to form a lamella which continues unbroken over the lacuna. Muscle attachments or sinew connections are only present in the intact bone, at some distance from the opening.
We have not yet been able to identify a common functional cause leading to fenestration in the neurocranium of cetaceans. It is neither an ageing phenomenon nor a pathological process. Nor is a generally valid explanation provided by mechanical muscle stress or intracranial pressure from the brain. Even the telescoping effect can only be partly held responsible for neurocranial fenestration. Apart from the temporal fossa, none of the cetacean lacunae can be homologised with the openings in the reptile skull. The lacunae in the cetacean braincase are not secondary phenomena. Apparently they are functionally insignificant, physiological structures. It is not impossible that they represent the rudiments of structures which were important at an earlier stage. The relatively large variability in terms of the position, shape and size of the fenestrations indicates that the process has not yet come to a halt.