No. 2.] HISTOGENESIS OF THE RETINA. 429 



with, and are enveloped by the pigment cells of the tapetum 

 nigrum. The line of perforation remains sharp and in this way 

 the membrana limitans externa is formed. 



In the amphibians the retina is completed in its center 

 before its area is very great, and its further growth is by gradual 

 addition to the periphery. In general, then, the ora serrata is 

 its growing point, and here its growth is by means of the same 

 steps as was the case in its center. Therefore, the histogenesis 

 can be studied in any growing retina, with the disadvantage, 

 however, of no pigmentation of the cells, which is of such great 

 value in the study of the first formation of the optic nerve. 

 Any Necturus embryo, during the first year of its life, will 

 show this periphral growth. The mitoses are also in all cases 

 in the outer nuclear layer, i.e., the side which corresponds to 

 the former outside of the body. This same process of addition 

 to the periphery is by no means peculiar to the amphibian eye, 

 but is present in all vertebrate, as well as many invertebrate 

 eyes. In the cephalopod eye, which is derived directly from 

 the ectoderm, the growing point is just beneath the bacilli 

 ("rod and cone"), or on the lens side of the retina. This is 

 exactly what we should expect, as it is this side which was ori- 

 ginally the free surface of the ectoderm. Here also the retina 

 completes its growth in its center and then becomes larger and 

 larger by addition to its periphery. Here also the plane of the 

 cell division is at right angles to the original ectodermal plane, 

 as is the case in the vertebrate eye. Not only is this true in the 

 squid, but also in various groups of arthropods. Watase, in 

 his excellent papers, has shown us much which brings the 

 invertebrate eye into line with the vertebrate. ^ He also 

 informs me that quite recently he has found that the eye of 

 Squilla grows in the same way. Lowne^ has also demon- 

 strated that the invertebrate eye grows at its periphery. 



Besides the methods of growth it is also possible to make 

 other comparisons between the vertebrate and invertebrate 

 eyes. The polarity of cells is identical and the chain of cells 



1 Watase, Studies from the Biol. Laboratory, Johns Hopkins Univ., Vol. IV, and 

 Johns Hopkins Univ. Circular, April, 1890. 



2 Lowne, Transaction of the Linn. Society, 1884. 



