130 MALCOLM LAURIE. 



lips of the invaginations have come together, but not fused. 

 The posterior layer of the invagination is visible as a thin layer 

 of cells (fig. 56, rtn'.), separated from the retina by a narrow 

 space. The vitreous layer {vit.) is distinctly marked as a 

 thickening of the hypodermis on the top of the head, the nuclei 

 in that region being elongated, but there is still a small space 

 separating it from the retina. By the time the embryo is 

 hatched the eye (PI. XVIII, fig. 57) has lost all connection 

 with the hypodermis at the point where it was invaginated. 

 The cells are long and deeply pigmented round their margins. 

 The pigment is not equally abundant throughout the whole 

 length of the cell, but five alternately more and less deeply 

 pigmented zones can be distinguished (fig. 58). The base of 

 the cells is most deeply pigmented, and their superficial ends 

 come next. The nuclei of the retinal cells lie in zone 4, 

 but I have been unable to find any trace of either rhabdomes 

 or phaospheres. I have also not been able to trace any migra- 

 tion of mesoderm cells among the retinal cells. The posterior 

 layer of the invagination can with difficulty be made out in 

 depigmented sections owing to the flatness of its nuclei, and it 

 is absolutely undistinguishable in sections from which the pig- 

 ment has not been removed. This posterior layer forms the 

 post-retinal membrane of the adult eye. The optic nerve is 

 beginning to grow out from the cerebral ganglion, but has not 

 yet come into connection with the eye. The hypodermis, im- 

 mediately in front of the eye, is formed of a single layer of 

 large transparent cells with faintly staining oval nuclei 

 (PI. XVIII, figs. 57 and 58, vit.). This vitreous layer is 

 covered by a thin cuticle exactly like that which covers the 

 rest of the surface of the body. The only sign of the formation 

 of the lens is a slight cupping of the vitreous layer at one point 

 (fig. 58). The hollow formed here is, however, not as yet filled 

 up by any cuticular substance, but the cuticle passes straight 

 over it. Hound the area, where the lens will form the hypodermis 

 is deeply pigmented. The cells are much smaller than those 

 of the vitreous layer, and their nuclei are irregular in shape. 

 The cells of the lateral eyes (PI. XVIII, fig. 59) are about 



