A study of mitotic index in differentiating lens of chick

Placodes of the chick embryo studied by SEM

The otic, the lens and the nasal placodes have been examined in chick embryos between stages 10 and 18 of Hamburger and Hamilton. At the stage when each placode first becomes visible conspicuous differences have been seen in the surface morphology between those cells which will invaginate and form the placode and those which will remain on the surface of the head, forming the epidermis. The differences become more pronounced with increasing development. The placode cells possess many surface projections whilst the epidermal cells do not. These differences in surface morphology are related to other differences which are visible in TEM sections, the placode cells being highly columnar and extending the full depth of the placode, whilst the epidermal cells are cuboidal or even squamous. This modification in cell shape of the placode cells is correlated with the presence of longitudinally orientated microtubules. The mechanism of invagination is discussed and evidence is presented which supports the idea that there is a migration of cells into the placode from one side. Such a phenomenon would help to explain the asymmetrical structure of the placode, including the presence of the overhanging lip.