48 THE FIRST DAY. [CHAP. 



thicker tlin.u in the rest of the pellucid area. It is apparently this median 

 thinning of the mesoblast which gives rise to the linear transparency seen in 

 specimens viewed with transmitted light. The hypoblast, it may be remarked, 

 is generally curved dovsmwards beneath the primitive streak and groove, though 

 not to the same extent as the epiblast. Thus the whole blastoderm is some- 

 what curved in this region. ' Immediately beneath the groove a kind of fusion 

 takes place between the epiblast and mesoblast, though on close examination 

 the line of junction between them can generally be made out. This apparent 

 fusion His (Ueher die Ersie Anlage des Wirbeltheirleibs) regarded as an event 

 of great importance, and gave the name of axis-cord to the part in which it 

 occurs. In fiesh specimens a narrow (opaque) streak can be seen running 

 down the centre of the groove; but it is not represented by any structure 

 ■which can be seen in sections. 



The chief events then which occur during the first twelve 

 hours of incubation are the estabUshment of the three layers 

 of the blastoderm, and the appearance of the embryonic 

 shield, of the primitive streak and of the primitive groove. 



5. From the 12th to the 20th hour. — During this period 

 the pellucid area rapidly increases in size, and from being 

 oval becomes pear-shaped. The primitive groove grows even 

 more rapidly than the pellucid area; so that by the 16th hour 

 it is not only absolutely, but also relatively to the pellucid 

 area, longer than it was at the 12th hour. The interval 

 between its end and the circumference of the pellucid area 

 continues to be greater in front than behind. 



At about the 16th hour, or a little later, a thickening of 

 the mesoblast takes place in front of the primitive groove, 

 giving rise to an opaque streak ending abruptly in front 

 against a semicircular fold, which appears at this time near 

 the anterior extremity of the pellucid area (Fig. 11), and 

 is known as the head-fold. In fresh specimens this streak 

 looks like a continuation from the anterior extremity of the 

 primitive groove ; but in hardened specimens it is easy to 

 see that the connection is only an apparent one. 



Along the new streak a groove (Fig. 11, m. c.) is very soon 

 formed, which, narrow in front, but widening very much 

 behind, embraces between its diverging walls the anterior 

 extremity of the primitive groove. This new groove, by the 

 conversion of which into a tube the medullary canal will be 

 formed, is known as the medidlary groove. 



On each, side of it the mesoblast is thickened, and the 

 surface of the blastoderm raised up in the form of two longi- 

 tudinal folds, known as the laintnce dorsales, or the medidlary 

 folds (Fig. 11, A). Immediately beneath the bottom of the 



