DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 



167 



neural tube appears in a cross section of a Chick embryo (fig. 107) 

 as a compact mass (Pv) consisting of many superposed small cells 

 which, as far as it is not divided up into separate blocks, is designated 

 as primitive-segment plate 

 or protovertebral plate. In 

 fig. 107 it is still connected 



O 



at the side by means of a 

 thin isthmus of cells with 

 the lateral plates, in whose 

 territory the middle germ- 

 layers are thinner and sepa- 

 rated from each other by a 

 fissure. 



In observing the blasto- 

 germ from the surface the 

 region of the primitive-seg- 

 ment plates, as is to be seen 

 in the posterior part of a 

 nine-days-old Rabbit embryo 

 (fig. 108), appears darkerthan 

 the region of the lateral plate; 

 so that the two are . dis- 

 tinguished from each other ; 

 one is stem-zone (stz), the 

 other parietal zone (pz). 



The development of the 

 primitive segments is ob- 

 servable in the Chick at the 

 beginning of the second day 

 of incubation, in the Rabbit 

 at about the eighth day. 

 Clear transverse streaks ap- 

 pear in the stem-zone at 

 some distance from the primi- 

 tive streak, about in the 

 middle of the embryonic 



fundament, both on the right and the left of the chorda and neural 

 tube (fig. 108). They correspond to transverse fissures, by means 

 of which the primitive-segment plates are divided into the small 

 and solid cubical primitive segments (uw). In the nine-days-old* 

 Rabbit embryo represented in fig. 108 these plates are resolved in 



