The Development of the AUigator 233 



quote directly Clarke's paper in the Journal of 

 Morphology (17) in description of these surface 

 views. 



STAGE I 

 Figures 2-2/ (Plates VI., VII.) 



The youngest embryo that we have for descrip- 

 tion is shown in Figures 2 and 2a. Of Figure 2 

 Clarke says: 



"The limiting line between the opaque and 

 pellucid areas is clearly marked, and within the 

 latter is a shield-shaped area connected by the nar- 

 rower region of the primitive streak with the area 

 opaca. The blastopore is already formed near 

 the posterior end of the shield. 



"A ventral view of another embryo of the same 

 age (Fig. 2a), seen from the ventral side, shows 

 that the blastopore extends quite through the 

 blastoderm, in an oblique direction downwards 

 and forwards, from the dorsal to the ventral side. 

 The thickened area of the primitive streak is here 

 very prominent. There is, too, the beginning of a 

 curved depression at the anterior end of the shield, 

 the first formation of the head-fold. " 



Transverse sections of this stage are shown in 

 Figures 2b-2f. 



Figure 2b, through the anterior region of the 

 blastoderm, shows a sharply defined ectoderm (ec) 

 which is composed of three or four layers of cells 



