292 JAMES ROLLIN SLONAKER 



THE PECTEN 



The development of the pecten is closely associated with similar 

 changes taking place in the chorioid. It first appears in the 

 seven-day sparrow embryo, a relatively late age when compared 

 to the chick embryo where the first appearance is in the fifth 

 day of incubation (Froriep). In cross-section it appears as a 

 small mass of cells, accompanied by a blood-vessel, projecting 

 from the region of the optic nerve entrance directly into the 

 vitreous body (fig. 60, P). This appearance is misleading, for, 

 as Nussbaum ('12) has shown in those animals with a pecten, 

 the arteria hyaloidea does not enter the eye at the level of the 

 optic disc, but through the chorioid fissure nearer the ora serrata. 



The structure now consists of an elongated mass of undiffer- 

 entiated cells, closely resembling those of the mesoderm sur- 

 rounding the eye with which it is more or less connected. A 

 small loop of a blood-vessel is carried in with these cells. In 

 other words, the cells of the mesoderm seem to be flowing into 

 the eye chamber, apparently through the chorioid fissure. In 

 this case it would occur without involving the walls of the optic 

 cup in any way. Some of the undifferentiated chorioid cells 

 may be included in this migration, but there is no evidence in 

 the sections at hand of any of the retinal cells' taking any part 

 in the formation of the pecten. The migrating cells and the 

 retinal cells are quite different in appearance and in their ability 

 to take the stain. The hyaloid membrane is pushed inward 

 by these migrating cells. If this membrane is considered a 

 part of the retina, then, to this extent, the retina plays a part 

 in the formation of the pecten, for the hyaloid membrane com- 

 pletely envelops the whole structure in the adult. In my opinion, 

 the pecten is formed from the mesoderm, and I cannot therefore 

 agree with Denissenko ('80), who claims that the pecten is 

 formed by the pushing in of the retina. In one of my speci- 

 mens, due to an artefact, a fold appeared in the retina. The 

 developing chorioid and pigment layers, together with some 

 mesodermal cells, extended into this fold. Since this happened 

 to occur at the optic disc, and the shape closely resembled that 



