The development of the pecten oculi, a structure peculiar to the avian eye, was studied by scanning electron microscopy (SEM) correlated with light microscopy (LM) in embryonic and adult chickens. The development of the chick pecten was divided into 4 phases: (1) formation of the primordial pecten (Hamburger-Hamilton's stages 27 to 29), (2) formation of the plate-like pecten (stages 30 to 34), (3) pleat formation and pigmentation (stages 35 to 37), and (4) bridge formation and high-vascularization (stage 38 to adult). The primordial pecten is formed entirely from the ectoderm by fusion of the inwardly-projecting edges of the optic fissure. The primordial pecten grows into a tall, thin plate rising from a broad base. The pecten begins to fold slightly at stage 35. The number of pleats increases rapidly, from 7 at stage 35, to 16 at stage 36, 18 at stage 37, and 19 to 20 at stage 40. The bridge begins to form at stage 38 by a swelling of the apical edge of the pecten and completes its development by the twentieth post-hatch day. Blood vessels appear first in the broad base of the plate-like pecten, then become more numerous and gradually extend into the pleats. The pecten becomes more vascular than cellular at stage 43, and it is highly vascularized in the adult. The pleat surface becomes conspicuously irregular with increased vascularization. The peripectinate cells, located on the pecten, are already present at stage 27.

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The present study was undertaken to characterize the structural changes and the glycogen distribution in the floor plate of the developing chick spinal cord. The floor plate consisted of ventricular zone by stage 19 (staged according to Hamburger and Hamilton, 1951). The marginal zone of this plate could be distinguished as a narrow border at stage 21. It increased progressively in thickness and was the same thickness as the ventricular zone at stage 26. It increased again in thickness from stage 38 onward. Glycogen appeared and subsequently disappeared in the floor plate of the chick spinal cord during development. Little, if any, glycogen appeared in the midportion of the floor plate at stage 19. The floor plate was weakly glycogen positive from the cervical through lumbosacral segments at stage 21. In the floor plate of the lumbosacral enlargement the glycogen staining was the highest and was maximal through stages 34-37. The floor plate of the cervical and thoracic segments except for the cervical enlargement continue to have low concentrations of glycogen. The glycogen staining of the floor plate began to decrease from stage 38, and at the same time neuroglial cells began to migrate into the marginal zone of the floor plate, later than in other regions of the cord. The glycogen staining in the floor plate was barely detectable at stage 40. The accumulation of the glycogen granules in the floor plate was found only in the radial glial cells.

The spinal cord of two tetraodontiform fishes, the Japanese file fish (Navodon modestus) and the panther puffer (Takifugu pardalis), are unusual among vertebrates in having a markedly abbreviated spinal cord with a long and flattened filum terminale. Only the rostral short part of the cord of both species is cylindrical; the greater part of the cord is markedly flat. The majority of the spinal nerve roots leave the short cylindrical part. The flattened part of the cord contains the central canal, myelinated nerve fibers, and a few motoneurons surrounding the cauda equina, and it is histologically similar to the filum terminale of amphibians and mammals. The spinal cords of other teleosts, the sun-fish and angler, also are abbreviated and possess a filum terminale and cauda equina. These orders possess an enormous head and short trunk. However, the correlation between this body form and an abbreviated cord is not causal, since the tetraodontiform species described here show ordinary body proportions. The spinal cord may be abbreviated in tetraodontiform fishes in general.

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