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This paper deals with the cytological development of the cells of the sensory line (CSL) of the pineal organ in Lacerta vivipara J. from the embryonic stage 30 on to the third day after birth. The general results are diagrammatically represented in figure 29. Only from stage 35 on, a diversity of sensory line cells can be distinguished. During ontogenetic development, in the photoreceptor cells (P) secretory activity is not observed. From stage 38 on, the photoreceptive poles of these elements show a differentiation of disks while, from stage 37 on, their basal processes make synaptic contacts with dendritic outgrowths originating from intra-epithelial sensory nerve cells. The embryonic secretory rudimentary photoreceptor cells (SRP) are characterized by an intensive secretory activity. Our quantitative analysis reveals a highly significant increase, during ontogenetic development, of the number of dense secretory granules, stored in the basal processes of the SRP. The photoreceptive poles of the SRP, just like those of the P cells, show the differentiation of disks from stage 38 on. Only just after birth, the outer segments of the SRP become rudimentary. From the absence of characteristic synaptic contacts between the basal processes of the SRP and dendrites of the sensory nerve cells it is concluded that, during ontogenetic development, the SRP become rudimentary as photoreceptivity is concerned. During the ontogenetic development of the pineal organ in Lacerta vivipara, morphological signs pointing to regression of its photoreceptor function are not very marked. In showing a photoreceptive differentiation of the outer segments of the SRP, the present investigation agrees with phylogenetic studies by other authors and supports the concept of the presence of a sensory cell line in the vertebrate pineal organ (Collin, 1969 b).