DEVELOPMENT OF THE DOG-FISH. 47 



ultimately becomes the pigment layer of the' choroid. J3y closure 

 of its mouth, the pit of involuted skin becomes a completely 

 closed sac. At the same time it becomes detached from the 

 external skin, which forms a continuous layer in front of it — all 

 traces of original opening being lost. There is thus left lying in 

 the optic cup an isolated mass of skin : this is rudiment of the 

 lens. The small cavity within it speedily becomes still smaller 

 by the thickening of its walls, until it disappears altogether. At 

 its first appearance, the lens is in immediate contact with anterior 

 wall of optic cup. In a short time, however, the lens is seen to 

 lie in the mouth of the cup, a space making its appearance 

 between the lens and anterior wall of optic cup. Walls on under 

 surface of optic cup are at first imperfect, as a gap here exists. 

 Through this gap mesoblast, in which eye is embedded, passes out 

 and forms vitreous humour, the gap subsequently closing. The 

 mesoblast immediately surrounding the eye forms the choroid 

 itself (pigmented layer of which we have seen to be already 

 formed), and also the sclerotic. The front portion of waljs of 

 optic cup, along with the, choroid, bend in front of the lens, and 

 forms the iris. The original wide opening of optic cups is then 

 narrowed to a smaller orifice — the pupil — and lens which lay 

 before in the open mouth is now closed in the cavity of the cup. 

 Superficial skin becomes transparent cornea. 



REPRODUCTION AND URINARY ORGANS. 



In embryo there are many coiled tubes lying on each side in 

 abdominal cavity, and opening into it by both ends. These 

 being somewhat similar to the .segmental organs of lumbricus: 

 may be called segmental tubes* A common segmental duct is 

 then formed on each side, which opens by one end into the 

 abdominal cavity, and by the other into the cloaca. Into this 

 common segmental duct each segmental tube comes to open by ' 

 one end. The common segmental duct splits into two — the 

 wolffian and mullerian ducts. If embryo is to become a male 

 the former remains and the latter atrophies, ; and conversely, if 

 embryo is to become a female, the mullerian duct persists and 

 the wolffian duct atrophies. The segmental tubes may be divided 

 into an anterior, middle, and posterior set. In female, cells 



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