252 



ZOOLOGY. 



and primitive hypoblast are connected by cells of inter- 

 mediate character. This point becomes the seat of very 

 active cell-division, and as the blastoderm extends the point 



becomes drawn out into a line, 

 which shows in surface view 

 as a streak the primitive streak 

 -Bowing to the greater thickness 

 of the blastoderm along it (figs. 

 127 and 128). 



Comparison with the frog 

 shows a striking series of re- 

 semblances between this primitive 



Fig. 127. SURFACE VIEW OF BLASTO- , -, , , , T _. . 



DERM, FIRST DAY OF INCUBATION, streak and the blastopore. Both 



are median and posterior; both 



mark the point where epiblast and primitive hypoblast are 

 connected by cells of intermediate character; both are 

 regions of active growth ; and (at a later stage) both are 

 enclosed by the neural folds, while traces of a neurenteric 

 canal are found in the primitive streak of the chick. We 

 may therefore regard the primitive streak as representing 

 the coalesced lips of the blastopore, whose actual cavity is 

 obliterated by the compression of the blastoderm by the 

 huge mass of yolk. 



7. Hypoblast and Mesoblast. These two layers are 

 formed by cell-division and differentiation in the primitive 

 hypoblast. A portion of the mesoblast also seems to be 

 directly formed from 

 the primitive streak 

 (fig. 128), but this 

 is in the extra- 

 embryonic region 

 only. A great deal 

 is formed indirectly 

 from the primitive 

 streak, since this latter gives rise to additional primitive 

 hypoblast, as well as additional epiblast. 



8. Neural Folds, Notochord, Coelom and Somites. In 

 front of the primitive streak the epiblast of the middle 



Fig. 128. TRANSVERSE SECTION THROUGH 

 TRIMITIVE STREAK. 



