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epiblast, and gives rise to the epidermis and otlaer organs, tlie innermost, which is only 

 one cell thick, is called the hypoblast, and gives rise to the intestine, the middle layer is 

 called the mesoblast, and gives rise to the bones, muscles, and blood vessels. In the 

 dorsal rudiment the same three layers are found, but the epiblast is here thickened into 

 a great keel which extends inwards and causes the projection of the rudiment; the 

 greater part of this keel ultimately separates from the extreme outer layer and forms 

 the brain and spinal cord. In the centre of the rudiment beneath the keel is a 

 cylindrical rod of cells which become converted into an elastic supporting structure, 

 the notochord. Eound the yolk, when it has been completely enveloped, there is a 

 layer of protoplasm containing nuclei, but not divided up into cells. The hypoblast 

 rests on this layer which is called the periblast. In the ventral region of the developing 

 embryo there is nothing but epiblast separated from the periblast by the flattened 

 cavity previously mentioned and called the segmentation cavity. The hypoblast bends 

 round and forms a straight tulje which lies in a depression in the periblast. This tube 

 is the intestine, and when first formed has neither anterior nor posterior opening, 

 neither mouth nor anus. The swelling of the anterior part of the epiblastic keel forms 

 the brain and causes the protuberance of the head. The sense organs are formed by 

 thickenings of the epiblast: these thickenings become hollow and globular and, sinking 

 into the interior part of the head (except the nasal organs), become connected with the 

 brain by the sensory nerves. The nasal organs become cup-shaped, their cavities 

 being open to the exterior by a single aperture each, which is ultimately divided 

 into two, the two nostrils of each organ. The heart is formed from mesoblast which 

 extends below the front part of the intestinal tube ; it opens posteriorly at first from 

 the segmentation cavity, but when the yolk is absorbed becomes connected with 

 veins. The inner part of the mesoblast forms the muscles and skeleton, the outer part 

 forms the fibrous layer of the skin in which the scales are produced. Eings of bone 

 formed round the notochord give rise to the vertebrae, while the bones of the skull 

 are formed from the mesoblast round the brain. The formation of the mouth and gills 

 takes place after hatching by the development of rods of cartilage between which clefts 

 appear placing the anterior part of the intestine in communication with the exterior. 

 The mesoblast on each side splits horizontally, its inner thinner layer remaining 

 attached to the intestine to form the muscles and fibrous tissue of the gut, while the 

 outer part forms the body muscles. The cavity thus formed is the body cavity, and at 

 the dorsal part of it are formed the kidneys and reproductive organs. The liver is 

 formed as an outgrowth from the intestine. The fins are merely folds of the skin, the 

 mesoblast of which gives rise to their muscles and bony rays. 



I have not succeeded in obtaining any specimens of the young sole in process of 

 metamorphosis ; the next stage in which I have met with it is that of a small fish 

 having in almost all i-espects the same structure as the adult. The smallest specimens 

 of this kind which I obtained were from ^ in. to f in. in length (12 to If) mm.). 

 One of them is represented in Plate XVI, Fig. 5, magnified 8^ times. The great 



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