358 



THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



ANTERIOR 



^t>4 



,' -^t 



^ 





It 



, ^f-i- 



AREA PELLUCIDA 



AREA OPACA 



EMBRYONIC SHIELD 



^1^'^^i^il^^ 





POSTERIOR 



Fig. 170. Early pre-primitive streak blastoderm of the chick. Blastoderm about 3.2 mm. 

 in diameter at this time. (After Spratt. '42.) 



DELAMINATING CELLS 



• • • 



SECONDARY OR TRUE BLASTOCOEL 



GERM WALL 



PRIMITIVE ARCHENTERIC SPACE 

 DELAMINATING CELL VITELLINE MEMBRANE 



EP I B L A ST 



ANTERIOR 



-GERM WALL 



c. 



SECONDARY BLASTOCOEL 

 ( 



P.OSTE RIOR 



PRIMITIVE ARCHENTERIC SPACE 



HYPOBLAST 



Fig. 171. Origin of the hypoblast (entoderm) in the avian blastoderm. (A) Median, 

 antero-posterior section of chick blastoderm. Entoderm arises by delamination from 

 upper or epiblast layer; possibly also by cells that grow anteriad from thickened posterior 

 area. (Based upon data supplied by Peter. '34, '38, and Jacobson, '38.) (B-D) For- 

 mation of the hypoblast (entoderm) from epiblast by a process of delamination in the 

 duck embryo. (Based upon data supplied by Pasteels, '45.) 



unincubated chick blastoderm is about 3 mm. in diameter, that of the duck, 

 about 2 to 3 mm. 



The most recent observations, relative to the formation of the second or 

 hypoblast layer, have been made upon the duck's egg (Pasteels, '45). In this 

 egg, Pasteels found that, at about nine hours after incubation is initiated, a 

 two-layered condition is definitely formed and that "the primary entoblast of 

 the duck is the result of a progressive delamination of the segmenting blastodisc 



