582 DEVELOPMENT. [CHAP. XLI. 



centre. This increases gradually, until it is about a line in diameter. 

 It is called the area pellucida. The darker and more granular part, 

 external to the area pellucida, is the area germinitiva, in which 

 may be observed a darker portion, which is the future area vascu- 

 losa, so called, because vessels are first observed in this situation. 

 The germinal membrane is composed entirely of cells, which result 

 from the original process of cleavage (page 574). These cells in- 

 crease at its margin; and in this way the germinal membrane 

 gradually extends itself so as to invest the entire yolk. 



Serous and Mucous Lamina. At an early period, the germinal 

 membrane manifests a tendency to split into two layers. These 

 are termed lamina. The upper one, which, in its extent, corre- 

 sponds to the area pellucida, is the serous lamina. The lower one, 

 which is more extensive, and which extends beyond the vascular 

 area, is called the mucous lamina. 



Reichert, however, in his beautiful investigations upon the deve- 

 lopment of the chick, has described the formation of the layers 

 from which the different organs are evolved in a somewhat dif- 

 ferent manner. 



The "Investing Membrane" and te Membrana Intermedia," of 

 Reicliert. The most superficial layer of cells of which the germinal 

 membrane is composed, which is usually known as the * ' serous 

 layer," has been termed by Reichert the investing membrane. It 

 extends, at its circumference, between the yolk and the vitellary 

 membrane by the apposition of new cells of the yolk to its peri- 

 phery, and is formed independently of the vascular system. The 

 rudiments of the nervous system are first seen in the form of a 

 line (primitive streak) immediately beneath the investing mem- 

 brane, and are developed from a stratum of ceils which has sepa- 

 rated itself from the surface of the yolk. Immediately beneath 

 this is another layer of cells which has been termed the mucous 

 layer of the germinal membrane, also the vascular layer, but Rei- 

 chert has well described it under the term membrana intermedia. 

 It is beneath the central organs of the nervous system, and in 

 contact with the chorda dorsalis. Lying between the nervous 

 centres on the one hand, and the mucous membrane on the other; 

 and extending beyond the limits of the area pellucida, the mem- 

 brana intermedia gives origin to the vertebral, cutaneous, and vas- 

 cular systems, and to the digestive system, with the exception of its 

 mucous membrane. In the development of the higher vertebrate 

 animals, according to Dr. Reichert, the membrana intermedia 

 performs a most important part. 



