EVOLUTION 



969 



venom gland seems a novel structure with startling lethal power, 

 but what is it but a specialised salivary gland ? It is usually regarded 

 as the upper labial gland, but some anatomists call it the parotid. 

 It has arisen by the specialisation of the ordinary type of salivary 

 gland; the secretion has become virulent; and it has come to be 

 enclosed in a strong fibrous sheath which is associated with powerful 

 muscles so that the poison is driven forward with great force to the 

 base of the fang. 



And as to the fangs, each is simply a tooth folded on itself during 



Fig. 164. 



Foetal Membranes of a Mammal. After Turner, zp, external ectodermic layer 

 with processes which effect preliminary attachment of the embryo to 

 the wall of the uterus; sz, the subzonal membrane, formed from the 

 union of the outer parts of the double amniotic folds; am, the amnion 

 proper; AC, the amniotic cavity containing fluid; E, the ectoderm of 

 the embryo; N, the mesoderm of the embryo; H, the endoderm of the 

 embryo, lining the archenteron or primitive gut; UV, the umbilical 

 vesicle or yolkless yolk-sac, opening into the archenteron; ALC, the 

 allantoic cavity, lined with endoderm, enveloped in mesoderm; al, the 

 inner wall of the allantois, which spreads in the space between the sub- 

 zonal membrane and the amnion proper. The embryonic part of the 

 placenta, which binds the embryo to the wall of the uterus, is formed 

 by an intricate union of allantois and subzonal membrane. 



early development, so that there is formed a groove or even a com- 

 plete canal, down which the poison passes to be injected into the 

 wound. In this case, though poison-gland and fang are both distinct 

 novelties, the transformation raises no great difficulty, for glands 

 and teeth are both very variable. Even the milk-glands of Mammals 

 are but highly specialised integumentary glands. 



(3) There is greater difficulty in the case of the allantois, which 

 forms a foetal membrane (respiratory, excretory, and nutritive) 

 around the embryo of Reptiles and Birds, and part of the placenta in 

 Mammals. It arises as a pocket from the under surface of the gut, 

 near its hind end, and it is the homologue of the cloacal bladder of 



