FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 905 



the embryo becomes entirely enclosed within the tissues of the uterus. These 

 phenomena are considered on pages 914, 920. 



2) Birds. The passage of the hen's egg down the oviduct has been studied 

 at various times from the time of Aristotle to the present. In its transportation, 

 the "naked yellow" or ovum becomes surrounded by an intricate association 

 of fibers, albuminous substance, membranes, and calcareous shell which form 

 a system of protective envelopes. As the egg of the hen passes posteriad in 

 the oviduct, it rotates slowly under the influence of muscular contractions and 

 the spiral arrangement of longitudinal folds of the mucous membrane lining 

 the oviduct. This rotation aids in the deposition of the membranes and albumi- 

 nous layers. 



a) Formation of the Chalaziferous Layer. The first coating of al- 

 bumen is deposited around the egg as it passes through the posterior portion 

 of the infundibulum (fig. 157). It is in the form of a sheet of mucin-like 

 fibers in the meshes of which is a dense albuminous substance. This capsule 

 of albumen is applied closely to the vitelline membrane of the ovum, and it 

 represents the membrana chalazifera, or chalaziferous layer (fig. 369A). (See 

 Romanoff and Romanoff, '49, pp. 137, 219.) 



b) Deposition of the Middle Dense Layer of Albumen. The egg 

 soon leaves the infundibular area of the oviduct and enters the albumen- 

 secreting region where a dense layer of albuminous material, the albuminous 

 sac, is deposited together with mucin fibers, the albumen being enmeshed in 

 the latter (fig. 369A). 



c) Formation of the Inner Liquid Layer of Albuminous Material 

 AND the Chalazae. As the egg continues its journey posteriad, it is rotated 

 upon the spirally arranged folds of the oviduct. This rotation twists the mucin- 

 like fibers in the inner portion of the dense albuminous layer, and it is believed 

 that this twisting motion squeezes the more fluid albumen out of the mucin 

 meshwork where it becomes deposited immediately around the chalaziferous 

 layer to form the inner liquid layer of albumen. At the same time, some of 

 the mucin fibers become twisted in opposite directions at the upper and lower 

 ends of the egg as the latter is rotated along the spiral folds of the oviduct. 

 These twisted fibers form a bundle at the anterior and posterior ends of the 

 egg and become attached firmly to the chalaziferous layer, reaching outward 

 into the dense albumen. These two bundles of twisted mucin fibers form the 

 chalazae, one chalaza being tied to the chalaziferous layer at the lower end 

 of the egg (i.e., the end occupying the more posterior position in the oviduct) 

 and the other lying attached to the chalaziferous layer at the upper end of 

 the egg (fig. 369A). 



d) Deposition of the Outer Liquid Albuminous Layer. As a result 

 of the resection experiments of Asmundson and Burmester ('36), one is led 

 to conclude that a considerable amount of the outer, watery, albuminous layer 

 which comes to surround the middle dense layer of albumen is deposited in 



