LIDS AND GLANDS; NON-MAMMALS 117 



ciliary epithelium. In the iris, the outer or anteriormost of the two layers 

 of epithelial cells eventually loses much of its pigment icf. Fig. 7g, p. 

 15) as it gives rise to the sphincter and dilatator of the pupil, which 

 are thus the only ectodermal muscles in the body. 



In the ciliary body, mesodermal cells differentiate into the ciliary 

 muscle fibers, and the anterior chamber widens and deepens greatly 

 through the erosion of tissue at the iris angle. From the ciliary epithelium 

 there develop the cuticular fibers of the suspensory ligament or zonule, 

 which are regarded collectively as the tertiary vitreous and which grow 

 axiad to gain secondary attachments to the lens capsule. The anterior 

 surface of the secondary vitreous then drops back to its definitive posi- 

 tion, its surface presented to the aqueous forming the anterior hyaloid 

 membrane; and the aqueous of the anterior chamber is free to spread 

 back into the posterior chamber and the canal of Hannover. With the 

 formation of the zonule, the main features of the eyeball are established. 



Lids and Glands — The lids arise as a circular fold of skin around the 

 front of the eye which closes in over the cornea, with its circular aper- 

 ture rapidly becoming a horizontal slit, thereby creating upper and 

 lower lids. The margins of these fuse together early in fetal life, opening 

 again much later — from a few days to six weeks after birth in mammals 

 which are born hairless and helpless. The time of reopening always 

 coincides closely with that at which the rods and cones have finished 

 their differentiation. That differentiation, it is interesting to note, can 

 be speeded up a couple of hundred percent by surgically opening the 

 lids of the newborn mammal and keeping it and its mother in a lighted 

 place. The various glands of the lids, the lacrimal and Harderian glands, 

 and the lacrimal drainage system are all ectodermal derivatives; but 

 their mode of development is unimportant to us here. 



Variations in Non-Mammals — Some major departures from the 

 above process, which occur in the different vertebrate groups, are men- 

 tioned briefly below and will be dealt with at some length subsequently, 

 in appropriate places. Others will be self-evident to the reader when, in 

 later chapters, he encounters mention of the loss or gain of some feature 

 by one group of animals or another. 



Lampreys : The epidermis and dermis of the skin are never fused to 

 the cornea to contribute respectively a corneal epithelium and a part 

 of the substantia propria. A patch of visual cells is already functional 

 in the primary optic vesicle (see Fig. 54c, p. 126) and persists as 'Retina 



