124 



EMBRYOLOGY. 



Fig. 87. Blastoderm of the Chick, incubated 33 hours, 

 after DUVAL. 



The area pellucida (A/) is surrounded with a portion of the 

 opaque area (c//). The fundament of the nervous 

 system is nearly closed in front and segmented into 

 the three brain- vesicles lib 1 , lib", hF; behind, the 

 medullary farrow (HI/) is still opeu. On either side 

 of the latter there are six primitive segments (us). 

 The posterior end of the embryonic fundament is 

 nccnpied by the primitive streak and the primitive 

 groove (pr). 



the primitive streak and its head-process, 



are closely in contact 

 at the bottom of the 

 groove, and indeed are in 

 places completely fused, 

 they together produce 

 in the pellucid area a 

 dark streak of sub- 

 stance, which is about a 

 millimetre long and'0'2 

 mm. broad. With the 

 earlier embryologists, to 

 whom it was already 

 known, we designate 

 this as the primitive 

 streak of the germ-disc. 

 In the vicinity of the 

 primitive streak there 

 are to be distinguished 

 in surface views, now 

 and during the following 

 stages of development, 

 some additional changes, 

 which are caused by the 

 beginnings of special or- 

 gans. In the first place, 

 there is to be seen in the 

 anterior region of the 

 area pellucida, and in 

 the direct continuation 

 of the primitive streak, 

 a narrow, dark streak of 

 cells, which has been 

 designated by KOLLIKER 

 as the head-process of 

 the primitive streak, 

 and which gradually in- 

 creases in length. Se- 

 condly, there appeal's an 

 increasing opacity (fig. 

 85) in the vicinity of 

 Avhich afterward stretches 



