BIRDS' EGGS 



261 



glands. In the isthmus itself there are tubular glands and 

 unicellular glands as in the albumen region. The isthmus 

 forms the shell-membrane, but also some albumen. 



The fourth region is called the uterus, in which there are 

 tubular and unicellular glands, but the former have a some- 

 what different appearance from those in the isthmus and 

 the albumen region. As Pearl and Curtis have shown, the 

 uterus not only forms the shell, but secretes thin albumen 

 which must pass through the shell and the shell-membrane by 

 osmosis. 



The fifth region is the vagina , from which tubular glands 

 are entirely absent, but the usual unicellular glands occur. 

 In the vagina these glands are perhaps concerned in secreting 

 the delicate outer shell- 

 cuticle and the pigment. 

 In other parts of the 

 oviduct the unicellular 

 glands are probably con- 

 cerned for the most part 

 in making a fluid or thin 

 albumen. 



Pearl and Surface (1909) 

 have made it clear that 

 the stimulus which excites 



the shell-secreting glands Fig. 42. — Diagrammatic 



of the oviduct (in the 



fowl at least) is mechanical 



rather than chemical in 



nature. Moreover, the 



formation of the shell is 



brought about by a strictly 



local reflex, and is not immediately dependent upon the 



activity of other portions of the reproductive system. 



A cross -section through the wall of the oviduct shows, 

 from the outside inwards, the following layers : — 



(i) A peritoneal investment, 



(2) A longitudinal muscular layer, 



(3) Connective tissue with blood-vessels. 



cioss-section 

 through the oviduct of a bird, i, 

 peritoneal investment ; 2, longitudinal 

 muscular layer ; 3 , connective tissue 

 and blood-vessels ; 4, circular mus- 

 cular layer; 5, another layer of con- 

 nective tissue ; 6, glandular layer ; 

 7, internal ciliated epithelium with 

 unicellular glands. 



