GASTRULATION IN VARIOUS CHORDATA 427 



position the involuted cells migrate laterally and anteriorly between 

 the hypoblast and epiblast. 



(c) In conjunction with the foregoing activities, epiblast cells (presumptive 

 mesoderm) from the posterior half of the epiblast of the pellucid area 

 migrate posteriad, converging from either side toward the median line 

 (fig. 204A-G). 



(d) These converging cells begin to pile up in the posterior median edge 

 of the pellucid area (fig. 204C), where they produce a raphe-like 

 thickening which marks the beginning of the primitive streak (fig. 

 204C-G). The beginning streak first makes its appearance at about 

 seven to eight hours after the start of incubation in the egg of the 

 chick (fig. 204C). 



(e) Once formed, the initial streak grows anteriad in the median line by: 

 (1) cell proliferation in situ, and by the addition of (2) converging 

 cells from the epiblast layer. 



(f ) Also, the primitive streak apparently grows posteriad by cell prolifera- 

 tion and the addition of converging cells. 



(g) When the migrating cells of the epiblast reach the primitive streak, 

 they involute and pass downward to the space between the epiblast 

 and hypoblast. From this new position they move laterad and anteriad 

 on either side of the midline, diverging to form a broad, middle layer 

 of mesodermal cells. 



(h) As the primitive streak grows anteriad in the epiblast, it eventually 

 approaches the presumptive pre-chordal plate and presumptive noto- 

 chordal areas. 



(i) The pre-chordal plate and notochordal cells then invaginate to form 

 the primitive pit; the latter represents a shallow or vestigial notochordal 

 canal, a structure strongly developed in reptiles and some birds, and 

 occasionally in mammals. 



(j) Notochordal cells from the notochordal crescent converge to the pit 

 area and probably pass downward in the walls of the pit, whence 

 they ingress and move forward in the median line (fig. 202A-G, J, K). 

 The definitive primitive streak is formed after about 18 to 19 hours 

 of incubation. At about 20 to 22 hours of incubation, the prospective, 

 notochordal material (e.g., the head process) has already invaginated. 

 At this time it represents a mass of cells in the median line intimately 

 associated with the neural plate ectoderm above the pre-chordal plate 

 cells and the entoderm below (fig. 2021, K). As the primitive streak 

 recedes posteriad (see p. 431 ), the notochordal material gradually sep- 

 arates from the surrounding, pre-chordal plate cells and also from the 

 neural plate material. Eventually the notochordal cells become a dis- 

 tinct median mass which elongates rapidly (i.e., undergoes extension) 

 as the nodal area and the primitive streak recede caudally (Spratt, '47). 



