50 BULLETIN OF THE 



The cells which I have thus far described in the differentiation of the 

 ommatidia are unquestionably ectodermal in origin. In stage F certain 

 nuclei appear which may have another origin. It will be recalled that 

 in stage C (Fig. 41), although the intercepting membrane is well devel- 

 oped, the retina and optic ganglion are still closely applied to each 

 other. In stage D (Fig. 45) the retina and ganglion have separated 

 enough to form an intervening space of considerable extent. In stage 

 E (Fig. 46) this space not unfrequently contains several nuclei. These 

 are smaller than the ectodermic nuclei in the retina, and of about the 

 size of those in the ganglion. Their chromatine differs from that of 

 both the retinal and ganglionic nuclei, in that it has the form of very 

 distinct particles which give the nucleus a decidedly granular appearance. 

 Moreover, these nuclei, which I believe to be mesodermic in origin, are 

 variable in shape, whereas the different kinds of ectodermic nuclei pos- 

 sess characteristic forms (Fig. 47). In stage F the space between the 

 retina and ganglion also contains a few mesodermic nuclei, and similar 

 ones are noticeable in the base of the retina (Fig. 51, nl. pig.). The 

 latter are different from the nuclei of the proximal retinula3 (Fig. 51, 

 nl. px.), and resemble so closely the nuclei which in the adult have been 

 described as belonging to the accessory pigment cells, that I believe 

 them to be the nuclei of those cells. 1 



From stage F to that of the fully grown lobster the changes in the 

 retina are, with one exception, rather insignificant. The parts of the 

 retina increase considerably in size, especially at the distal end of 

 the ommatidium, and additional pigment is deposited in the distal and 

 proximal retinul^e. The only change of importance which the eye 

 undergoes before full maturity is reached is a rearrangement of the 

 ommatidia. It will be remembered that up to stage F the ommatidia 

 were so arranged in relation to each other that the resulting corneal 

 facets were hexagonal in outline. In the adult lobster the facets are 

 square. The change from the six- to the four-sided facet is effected, 

 I believe, by a partial slipping of rows of ommatidia on each other. 

 Imagine, in Figure 55, a row of ommatidia extending in the direction 

 of the arrow and on either side, and parallel to this another row. Only 

 one ommatidium in each of these two lateral rows is given in the figure. 



1 When the preliminary notice of this paper was written, I was somewhat in 

 doubt as to the origin of the accessory pigment-cells. I had not then had the op- 

 portunity of studying stage F, and I was of opinion that these cells were probahly 

 of ectodermic origin. I believe that the evidence which is now at hand indicates 

 that they are cells derived from the mesoderm. 



