\ 



CHAP. XLI.] FIRST TRACE OF EMBRYO. 58.-J 



About the sixteenth hour, between the external and internal 

 layers of the germinal membrane a new deposition of cells takes 

 place, from which the first traces of vessels are produced. This is 

 the middle layer of the germinal membrane, commonly known as 

 the vascular layer. The vessels are not formed, in the first place, 

 close to the embryo, but at some distance from it, outside the area 

 pellucida, so that now the area pellucida is surrounded by the area 

 vasculosa; and this again by the area vitellina. As the ves- 

 sels increase in number and 

 size, they spread outwards over 

 the area vitellina, and inwards 

 over the area pellucida, until 

 they join the large trunks 

 connected with the heart. A 

 little earlier than this, about 

 the fourteenth hour, in the 

 central part of the long axis of 

 the clear oval area pellucida, 

 which lies in the transverse 

 axis of the egg, appears a line 

 thicker at one extremity than 

 the other. This is the first 



iwnr>o r\f +lio AmVifwrt onrl ia Portion of germinal membrane about the sixteenth 

 trace Ot tlie em Dry O, and. IS hour of incubation, a. Germinal membrane, b. Area 



called the primitive streak or $- c - Area pellucida - * Nota primitiva or 

 the nota primitiva. 



About the third day of incubation, on each side of the primitive 

 streak, appear two ridges or crests, which are termed lamina dor sales. 

 These gradually arch over, and at length enclose the brain and cord. 

 The chorda dorsalis occupies a position underneath the spinal cord, 

 and appears as a thin gelatinous thread, which corresponds to 

 the axis of the bodies of the vertebrae, which latter, however, are 

 not formed from the chorda dorsalis, but are developed from a 

 double row of four-sided white spots, or vertebral plates, arranged 

 symmetrically on each side of the central chorda dorsalis. Passing 

 downwards from the dorsal laminae, on each side are the ventral 

 lamina, which tend to enclose a space below the chorda dorsalis, 

 like that enclosed by the dorsal lamina above it. The former con- 

 tains the large vessel of the trunk (Fig. 277). The latter the 

 central organs of the nervous system. In this way are produced 

 the hamal arch and the neural arch of the vertebrae. 



Soon the embryo, and that portion of the germinal membrane 

 immediately connected with it, become somewhat raised; the 



