THE ORGANS OF THE OUTER GERM-LAYER. 487 



become united throughout their whole extent, their epithelial invest- 

 ments fusing with each other. In Man the concrescence begins in 

 the third month, and usually undergoes retrogression a short time 

 before birth. But in many Reptiles (Snakes) the closure is perma- 

 nent. Thus a thin transparent membrane is formed in front of the 

 cornea. 



In Man during the concrescence of the eyelids there are developed 

 at their margins the Meibomian glands. The cells of the rete 

 Malpighii begin to proliferate and to send into the middle connective- 

 tissue plate of the eyelid solid rods, which afterwards become covered 

 with lateral buds. The glands, at first entirely solid, acquire a 

 lumen by the fatty degeneration and dissolution of the axial cells. 



At about the time of the development of the Meibomian glands, 

 the formation of the eyelashes takes place ; this corresponds with the 

 development of the ordinary hair, and therefore will be considered 

 along with the latter in a subsequent section of this chapter. 



In most of the Vertebrates there is associated with the upper 

 and under lids still a third, the nictitating membrane or membrana 

 nictitans, which is formed at the inner [median] side of the eye as 

 a vertical fold of the conjunctiva. In Man it is present only in a 

 rudimentary condition as plica semilunaris. A number of small 

 glands which are developed in it produce a small reddish nodule, 

 the caruncula lacrymalis. 



The lachrymal gland is an additional auxiliary organ of the eye, 

 which is destined to keep the sac of the conjunctiva moist and the 

 anterior surface of the cornea clean. In Man it is developed in the 

 third month through the formation of buds from the epithelium of 

 the conjunctival sac on the outer side of the eye, at the place where 

 the conjunctiva of the upper lid is continuous with that of the eye- 

 ball. The buds form numerous branches, and are at first solid, like 

 the Meibomian glands, but gradually become hollow, the cavity 

 beginning with the chief outlet and extending toward the finer 

 branches. 



A special efferent lachrymal apparatus, which leads from the inner 

 angle of the eye into the nasal cavity, has been developed for the 

 removal of the secretions of the various glands collected in the 

 conjunctival sac, but particularly the lachrymal fluid. Such an 

 apparatus is present in all classes of Vertebrates from the Amphibia 

 upward ; its development has been especially investigated by BORN in 

 a series of researches. 



In the Amphibia it begins to be formed at the time the process of 



