FORMATION OF THE YOUNG ANIMAL 



69 



the 

 the 



ocellus or eye-spot appears first, and the rest are added one by 

 one until the full number are reached. 



The first appearances connected with the formation of the 

 eye take place in the cellular layer just beneath the chitiuous 

 exoskeleton. This layer, called the hypodermis, plays an im- 

 portant part in the organisation of the animal. It forms the 

 inner layer of what we may call the skin of the animal, and the 

 cells of which it is composed secrete the chitin of which the 

 shell or exoskeleton of the animal is composed, and which is 

 moulted every year. 



The first process in the formation of the eye-spot is 

 thickening of the hypodermis beneath the chitin, just in 

 place where the eye will come. 

 At the same time the cells of 

 this thickened mass of hypoder- 

 mis secrete a quantity of pigment 

 of a dark red brown colour. 

 Next the cells of the thick mass 

 of hypodermis begin to separate 

 from one another in such a way 

 that a vesicle is formed. This 

 vesicle is hollow inside, and the 

 thick walls are formed from the 

 cells of the thickened hypodermic 

 mass. This can be seen from 

 rig. 44, which represents a 

 section through an ocellus when 

 it is partly formed. From this 

 vesicle the eye is formed. 



The wall of the vesicle near- 

 est the exoskeleton gives rise to ^^^ 44. -Section through eye wh 



S.TO.U- 



first 



forming : Hyp, hypoderraihi ; L71, lens ; 

 1^. W. V, front wall of optic vesicle, ; 

 h.w.i\ back wall of vesicle ; c«jj, 

 capsule. 



the lens of the eye, while the 



other walls of the vesicle form 



the retinal parts of the eye. 



The cells from the brain grow out 



and form the optic nerve connecting the retina with the brain. 



The whole eye spot is covered internally by a thin membrane, 



formed not from the hypodermis but by cells from the inside of 



the body (mesoblast cells). 



In the Chilognatha, the first Order of Myriapods, the young 



