58 VERTEBRATE PHOTORECEPTORS 



in the initial protoplasmic bud and regarded them as mito- 

 chondria. This author thought that the transverse stria- 

 tions of the external segment and its cleavage into discs 

 were the expression of its mode of construction from mito- 

 chondria. The granules which he has called mitochondria 

 bear a striking resemblance to those observed in the Am- 

 blystoma retina, yet in the latter, the eyes were fixed with 

 the aid of 5 per cent acetic acid, which is a mitochondrial 

 solvent. Our observations suggested that the heavily stained 

 granules are elaborated in the cytoplasm of the visual 

 cells and that the globule comes about through the partial 

 vacuolization of the cytoplasm. 



The development of the rat retina (Detwiler, 1932) 

 offers some interesting features. The eyelids of the rat do 

 not open until about 13 days after birth. At birth the retina 

 has undergone very little differentiation. There is a broad 

 zone of nuclei, corresponding somewhat to the outer neuro- 

 blast ic layer of a 15-17 mm. human embryo. This zone is 

 characterized, during the first two days, by the presence of 

 many mitotic figures along the external border adjacent to 

 the epithelial pigment layer. During the third and fourth 

 days, mitotic figures become less in the fundus, but they are 

 still numerous as the or a is approached. The beginning of 

 visual cell formation is evident on the fourth day by the 

 presence of very short protoplasmic extensions into the 

 cavity of the primary optic vesicle. These measure 1.5 to 

 2.2 ju in length, and are seen only at the posterior pole of the 

 eye. Also during the fourth day one can see the first indica- 

 tion of some separation of the broad nuclear zone into two 

 layers — presumably the future external and internal 

 nuclear layers. During the fifth day, mitotic figures can 

 still be seen in the region of the fundus, but by the sixth 

 day they have all disappeared, and they become less numer- 

 ous also in the region of the ora serrata. On the sixth day 

 the external nuclear and internal nuclear layers are com- 

 pletely separated by the formation of a definite external 

 molecular zone. Rod growth proceeds very slowly. At this 



