EXISTING ORGANISMS— EMBRYOLOGY 



45 



brain and spinal cord, while the ventral part forms anterior and 

 posterior connection with the exterior and gives rise to the ali- 

 mentary tract and its derivatives. 



The Foetal Membranes. Associated with terrestrial life the 

 vertebrates above the Amphibia develop a remarkable series of 

 structures called the foetal membranes, which compensate the 

 differences between the heavy water as a medium in which the 

 lower forms de- . 



velop and the light, 

 dry air which sur- 

 rounds the eggs of 

 the reptiles and 

 birds (Fig. 25). 

 Two of these mem- 

 branes are formed 

 in the hen's egg by 

 the growth of folds 

 from the layers of 

 tissue surrounding 

 the young embryo. 

 The folds consist 

 of an outer layer 

 of ectoderm and 

 an inner layer of 

 mesoderm. By a 

 process of involu- 

 tion, similar to the 

 formation of the 

 neural tube, these 

 folds unite to form 



B 



A mnion folds 



A mnion 

 Chorion 



Chorion 



Fig. 2.5. — Diagram illustrating the development of 

 the foetal membranes Al, allantois; Am, amniotic 

 cavity; Ys, yolk sac. The chorion of this figure is 

 the serosa of the text. (After Gegenbaur in McMur- 

 rich; taken from Arey's Developmental Anulomy with 

 the permission of the W. B. Saunders Company.) 



two layers covering the embryo; the outer consists of ectoderm 

 outwardly and of mesoderm inwardly, and is called the serosa, 

 while in the inner, called the amnion, the order is reversed. 

 The amnion is filled with fluid which protects the delicate 

 tissues of the growing embryo in every way as effectively as 

 the ocean protects the developing embryo of a fish. As the 

 outer edges of the cap of growing tissue extend, they finally 

 meet to enclose the yolk. The serosa is then a hollow sphere, 

 which must be involved in any contact with the outer world. 

 Inside of it another sac is then completed, lined with endoderm 



