512 STANLEY C. BALL 



this section. Moreover, the second eye of this embryo was in 

 the same stage. 



The fate of these undifferentiated cells will be treated later. 

 The eyes are at first separated from the ectoderm by three or 

 four rows of cells. Partly as a result of increase in size which 

 the eyes undergo, and partly because the cells among which 

 they lie are gradually withdrawn to form other tissues, the eye 

 cells shortly assume a position so close to the ectoderm that only 

 one layer of nuclei intervenes. The left eye in figure 28 (from 

 a frontal section through the eyes and brain) has become fi- 

 nally located. Apparently at this stage the optic cells are entirely 

 independent of the brain. Unusual turgidity characterises the 

 embryonic eye; it retains a spherical form in spite of the pres- 

 sure which must be exerted upon it by the surrounding cells. 



A series of stages in the development of the eye is illustrated 

 by figures 30 a:nd 31. All were drawn with the aid of a camera 

 lucida in order to bring out the gradual increase in the size. 



During the period of greatest activity of the eye cell its nucleus 

 is prominent, with a fine chromatin reticulum; a nucleolus has 

 not been detected. It has been mentioned in the section dealing 

 with the cleavage of the egg that the active blastomere nuclei 

 tend to assume an amoeboid form. So in the eye cell the nucleus 

 becomes to a lesser extent irregular during the deposition of 

 melanin granules. 



Two types of melanin spherule deposition have been observed. 

 The more usual is that wherein the granules arise one at a time. 

 While those first formed are increasing in size new ones appear 

 in the cytoplasm (fig. 30). The earlier melanin bodies are 

 formed close to the lateral surface of the nucleus. Hence it 

 might be inferred that, being laid down under the influence of 

 the latter, these would soon be pushed outward by new ones 

 to be formed in the same proximity to the nucleus. Such a 

 sequence, however, has not been observed; the new spherules 

 arise as often on the outside of the group as on the inner nuclear 

 border. Indeed, after the process has gone on for some time 

 practically all the smaller granules are seen to lie on the side 

 opposite the nucleus. 



