Reproduction 



291 



Embryonic and Fetal Membranes 



In the description (pp. 240-243) of the reproductive arrangements in 

 vertebrates, a general account of the embryonic membranes, amnion, 

 chorion, and allantois, of reptiles, birds, and mammals was given. The 

 foregoing account of the origin of the germ layers and the shaping up 

 of the embryonic body now makes it possible to appreciate the manner 

 of formation of these membranes in terms of germ-layers. 



All eggs are invested by protective coverings which are either pro- 

 duced by the ovum itself or secreted about the egg by the oviduct. 

 Such membranes consist of material which is not cellular and not in 

 any sense living. They have merely passive functions. The amnion, 

 chorion, and allantois are produced by the germ-layers at a relatively 

 advanced stage of the embryo. They are constituted of living cellular 

 material and are actively concerned with such important functions as 

 nutrition, respiration, excretion, and circulation. 



The amnion and chorion are simultaneously produced by an up- 

 rising fold of the somatopleure (ectoderm accompanied by mesoderm) 



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Fig. 240. (A) Dorsal view of the head of a chick embryo after about 33 hours' 

 incubation. The head-fold of the amnion covers a small anterior portion of the 

 embryo's head. ( X 32.) (B) Section of the head of an embryo at a similar stage, 

 the plane of section indicated by S-S in (A). (A) Auditory invagination of ecto- 

 derm; (AM) head-fold of amnion; (EC) ectoderm; (EC) superficial ectoderm of 

 head; (EN) endoderm; (H) heart; (HE) endothelial cavity of heart; (MH) hypo- 

 mere mesoderm which gives rise to the myocardium, epicardium, and pericardium 

 (see p. 286); (MS) mesodermal somites; (NC) notochord; (NT) neural tube; 

 (O) optic vesicle; (P) pharynx; (1) forebrain; (2) midbrain; (3) hindbrain. (After 

 Duval. Courtesy, Neal and Rand: "Comparative Anatomy," Philadelphia, The 

 Blakiston Company.) 



