A motor designed to rapidly spread monolayers of cells across a slide for morphologic studies is a:

The device designed specifically for rapidly spreading monolayers of cells across a slide for morphologic studies is the cytocentrifuge. This instrument utilizes centrifugal force to deposit and concentrate cells onto a slide in a uniform layer, which is ideal for microscopic examination. By utilizing a cytocentrifuge, the cells maintain their morphology and arrangement, allowing for accurate assessment and analysis.

In contrast, other types of centrifuges such as horizontal-head and fixed angle-head centrifuges are primarily used for separating components of mixtures based on density, typically in larger volumes, and do not provide the same level of control over cell deposition on slides. An ultracentrifuge, while capable of achieving extremely high speeds for separating small particles, is not suited for preparing monolayers of cells on slides. Therefore, the cytocentrifuge is specifically designed to meet the needs of morphologic studies, making it the correct choice in this context.