ICj0 CLARKE. [Vol. V. 



when the shell was removed ; the egg was kept fully exposed 

 to the air of the room, where the temperature was nearly 90 

 Fahrenheit ; line 2 represents the belt as it was sixteen minutes 

 later ; and line 3 shows the increase at thirty-seven minutes after 

 line 2 was made. The exposure to the air, then, increases the 

 rapidity of that change in the shell membrane by which it be- 

 comes chalk-white. As this zone increases, it often becomes 

 considerably widest at one place, and it is underneath there that 

 the embryo is found. I would suggest that this change is pro- 

 duced by the gaseous interchange taking place through the 

 membrane, and which still further adapts it to its part in the 

 respiratory function. Certain it is that the change occurs in 

 the region of the embryo and around the yolk, and enlarges 

 as the embryo enlarges. Moreover, better respiratory arrange- 

 ments are necessary in eggs which are buried some 40 cm. 

 deep. Rathke (18), in his careful description of the shell 

 membrane of Laccrta agilis, states that it is more complex than 

 that of the adder and is composed of four parts ; of these he 

 reckons the shell as the first or outermost ; (2) a thin, transpar- 

 ent layer which, fitting close to the shell, becomes uneven on 

 its outer, but is perfectly smooth on its inner, surface ; it is 

 easily stripped off from the underlying membrane ; (3) a most 

 delicate layer, clear as glass, showing no trace of either fibres 

 or granules, and is elastic and easily thrown into thin folds ; (4) 

 a much thicker membrane not easily torn. At each end of the 

 egg this innermost membrane in a moderately large area forms 

 a segment of a hollow sphere, which has quite another character 

 than the median, very much larger part. This is non-transpar- 

 ent, white of color, and contains many very small granules. The 

 end pieces, on the contrary, were transparent, colorless, without 

 granules and thinner. This description is of the shell mem- 

 brane of an egg removed from the oviduct. It is especially 

 interesting in that a median, opaque white zone is described, 

 like that occurring in the alligator's egg. It appears much 

 earlier, however, if it is formed while the egg is in the oviduct. 

 It is not stated whether this zone shows also in the shell ; and 

 as the zone will quickly grow when the membrane is exposed 

 to the air, I cannot feel sure that the zone is normally devel- 

 oped in the membrane while the egg is still within the oviduct. 

 Fig. 5, PI. IX, is of an egg in the fifth week of incubation ; 



