586 DEVELOPMENT. [CHAP. XLI. 



The primitive streak is formed in the serous or animal layer of 

 the germinal membrane, beneath the investing membrane of Rei- 

 chert. BischofT has shown, that in the mammalian ovum the 

 germinal membrane becomes divided into two laminae, as in the 



Fig. 276. 



Portion of the germinal membrane of the bitch's ovum, showing the area pellucida and first 

 traces of the embryo, after Bischoff. a. Germinal membrane; b. area vasculosa; c. area 

 pellucida ; d. laminae dorsales ; e. primitive trace. Magnified ten diameters. 



chick. The external lamina is the serous, or animal layer, and the 

 internal one is the mucous or vegetative layer. The middle layer, 

 or membrana intermedia, described by Reichert, has already been 

 alluded to in page 582, but according to Bischoff is not to be de- 

 tected in the mammalian ovum. 



Formation of Dorsal and Ventral Lamina. In a short time, the 

 cephalic extremity becomes enlarged, and the entire embryo of a 

 guitar-shape in its general outline. As soon as the primitive 

 groove is formed, two oval folds are seen to rise up on each side of 

 it from the serous laminae (d. fig. 276). These are the lamina 

 dorsales, which gradually approximate; and the groove between them 

 becomes converted into a canal which contains the central organs 

 of the nervous system. 



Reichert supposes that the lamina? dorsales actually take part in 

 the formation of the central parts of the nervous system; but 

 Bischoff considers that they represent the dorsal portion of the 

 embryo, and that the nervous system is developed from their lower 



