7 
opment of the embryo. Now on the contrary other organs, for 
instance the senses, appear much later in the embryo, and 
generally fade away in the decrepitude of age, often long before © 
death. Still shorter, and confined only to the best part of life, 
at least with mankind, is the activity of the reproductive 
organs, but the organs of far the shortest duration are found 
with Vertebrates in the embryonic period. The placenta of 
the young Mammalia, and the yolk placenta of the Sharks, are 
confined to the short period of the gestation of the young in 
the womb of the mother. The allantois of the Mammalia, 
Birds and Reptiles proper, the organ of respiration of the 
embryo as long as it is enclosed in the membranes of the egg, 
disappears as soon as the embryo ishatched. The interior gills of 
the tadpol es of our frogs, and their only locomotive organ, their 
strong tail, last only through their fishlike period of life in 
water, some weeks or months; the exterior gills of the same 
only for some days. Thus we may have a series of the duration 
of the organs of one individual. But there is hardly in an 
Vertebrate, another organ, which is so transient as the tooth of 
the hatching lizards and snakes, previously described. Its 
only function is to cut open the eggshell, which it may perform 
in the time of a second, and soon after it drops. 
EXPLANATION OF THE PLATE. 
Fic. 1 Head of the hatching Black Snake, (Coluber con- 
strictor, Linn.) a. Snout, or rostral plate. 
b. Egg-tooth. c. Slit of the windpipe. d. Tongue. 
Fie. 2. The Egg-tooth with a part of the intermaxillary bone. 
(Natural size.) 
Fie. 3. The same seen from above. (Magnified 125 times.) 
b. The tooth. c. Intermaxillary bone, showing the little 
bone cavities, or bone cells. 
d. The contour of the intermaxillary bone, appearing through 
from the lower side. e. Nutritive canals of the dentine. 
Fig. 4. The same seen from above. (Magnified 350 times.) 
a. Anterior excavation of the tooth. 
b. Ramifying canals of the cement. c. Simple canals. 
d. Pulp of the tooth. e. Margin of the bowl-like swelling in 
the middle. 
f. Intermaxillary bone, with its bone-cells. 
Fig. 5. The tooth alone, in profile. (Magnified. ) 
Fig. 6. Tissue of the eggshell, showing the crossing fibres, a 
and b. (Magnified 350 times. ) 
Fic. 7. a. b. c. d.e. fi, several stages of the fibre-bulbs, taken 
from the fresh egg shell of Tropidonotus natrix, Kuhl. 
