Initial arealization of the developing forebrain is controlled by four patterning centers, the best studied of which is Anterior Neural Ridge (ANR). ANR is located at the anterior junction between neural and nonneural ectoderm.
When neural plate folds fuse at the anterior margin of the forebrain, ANR becomes Comissural Plate (CoP). ANR/CoP produce and release several members of the vertebrate FGF family. FGF molecules establish gradient of transcription factors Emx2 and COUP-TFI by repressing their expression in a dose-dependent manner.
Emx2 imparts posterior-medial area identities on cortical progenitor cells in which it is expressed, e.g. primary visual cortex area V1. COUP-TFI acts predominantly by repressing the identities of frontal/motor cortical areas within its expression domain in parietal and occipital cortex.
Imagine a brain with massive expansion of frontal areas, including motor areas accompanied by substantial reduction in the sizes of the three primary sensory areas, which become compressed to the caudal pole of the cortical hemisphere.
Which of the following defects of embryonic development most likely caused this phenotype?