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been made on the embryo of the chick, the author demonstrates its 

 mode of evolution, and also the mode of development of the various 

 layers of which this membrane is formed. They commence at the 

 early period of the thirty-third hour of incubation, at which time 

 the cephalic extremity of the embryo presents a slight protrusion of 

 its walls, which by the thirty-sixth hour is very considerably in- 

 creased, having become more elongated and protruded outwards, 

 presenting a somewhat dilated end, and being somewhat constricted 

 at its connection with the anterior cerebral cell from which it arises. 

 This is the first indication of the development of this membrane. 



At the forty-sixth hour this protrusion (which the author calls 

 the optic vesicle) was still more distinct, and the cavity in the cere- 

 bral cell, from the wall of which it arises, svas well seen, and it was 

 observed to communicate with the cavity of the optic vesicle which 

 was also hollow. This description of the mode of development of 

 the retina the author considers as confirmatory of the observations 

 made by Baer, but not in accordance with that given by Wagner 

 or Huschke. 



The author then proceeds to detail very minutely the consecutive 

 stages of the development of the retina and parts in immediate con- 

 nection with it, until the seventh day, when he states that on making 

 a section of the eye, it was separated from the other tunics as a per- 

 fectly distinct layer. The optic nerves were also now completely 

 formed, being united to form the chiasma, and passed inwards in 

 the direction of the under surface of the corpora quadrigemina. 



The author in the next place proceeds to consider the develop- 

 ment of the various layers of the retinal membrane, a point which 

 appears not to have been previously noticed by any physiologist. 

 This membrane on the eighth day of incubation can be seen, by the 

 naked eye, distinct from the other tunics. Its choroidal surface is 

 composed of a mass of globular nuclei about the size of the red cor- 

 puscles of the blood, which form at this period about one-half the 

 entire thickness of the retina, the deeper surface consisting of some 

 fine granular matter and a mass of pale and delicate nucleated cells 

 similar to those found surrounding the fibrous lamina in the normal 

 structure of the membrane. 



The "Membrana Jacobi" is first observed on the thirteenth day 

 as a fine pale granular stratum which covers in the globular nuclei 

 already described. In this, at about the fifteenth day, some brilliant 

 yellowish granules are imbedded ; they vary in size from the oOOOtli 

 to the 8000th of an inch, and around them a delicate cell-wall is 

 traced ; they soon acquire an oval shape ; then become more elon- 

 gated ; and about the eighteenth day the almost perfect rods are 

 formed. They are now disposed in an imbricated manner, and their 

 nuclei, which are of a bright yellow colour, are placed generally at 

 the apex, but sometimes in the middle of the rods. On the twenty- 

 first day this membrane is similar to what is seen in the full-grown 

 bird. 



The first trace of the "fibrous lamina" is seen between the four- 

 teenth and fifteenth days, as a fine pale granular lamina marked by 



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