260 



STUDY OF PIG EMBRYOS. 



the retina and lens will increase during the following stages and will become oc- 

 cupied by a very clear tissue containing a minimal number of cells. This clear 

 tissue is the commencement of the vitreous humor. Between the lens and the 

 overlying ectoderm the mesenchyma has begun to penetrate. This mesenchyma 

 will ultimately furnish the connective tissue of the cornea and of the iris. 

 About the eyeball as yet there is no distinct condensation of tissue such as will ap- 

 pear later to develop the anlages of the choroid and scleral coats of the eyeball. 



mes : /:-: 



Ec, Epidermis. 



tu-v 



Fig. 149. — Rabbit Embryo of Thirteen Days ; Section ok the Eyb. 

 L, Lens, mes, Mesenchyma. A 7 , Anlage of optic nerve. /', Pigment layer. 

 tu.v. Tunica vasculosa lentis. 



R, Retina. 



In the pig of 24 mm. (Fig. 150) we encounter a marked advance in the differ- 

 entiation of all parts of the eye. Above and below the eye the anlages of the eye- 

 lids, L.sup, L.inf, ha ve appeared. The anlage is at this stage merely a projecting 

 fold of the ectoderm filled with mesenchyma and extending a short distance over 

 the projecting cornea. The folds will continue to grow until the eyelids meet in 

 the middle of the eye, covering it completely. The ectoderm of the two lids where 

 they meet unites. The union of the two lids occurs in all mammals, and in some 



